Simple MP3 player that uses Tomas Mikula's excellent EasyBind library (https://github.com/TomasMikula/EasyBind). (You will need EasyBind v1.0.0 to run this.)

MP3Player.fxml

<?xml version="1.0" encoding="UTF-8"?>

<?import java.lang.*?>
<?import javafx.geometry.*?>
<?import javafx.scene.control.*?>
<?import javafx.scene.control.TitledPane?>
<?import javafx.scene.layout.*?>
<?import javafx.scene.layout.BorderPane?>

<BorderPane xmlns:fx="http://javafx.com/fxml" fx:controller="mp3player.MP3PlayerController">
  <bottom>
    <VBox alignment="CENTER" prefHeight="-1.0" prefWidth="-1.0">
      <children>
        <HBox alignment="CENTER" maxWidth="-1.0" prefHeight="-1.0" prefWidth="-1.0">
          <children>
            <ProgressBar fx:id="currentTimeIndicator" maxWidth="1.7976931348623157E308" prefWidth="-1.0" progress="0.0" HBox.hgrow="ALWAYS" />
          </children>
          <padding>
            <Insets bottom="5.0" left="15.0" right="15.0" top="5.0" fx:id="x2" />
          </padding>
        </HBox>
        <Label fx:id="timeLabel" alignment="CENTER" text="Label" />
        <HBox alignment="CENTER" maxWidth="-1.0" padding="$x2" prefHeight="-1.0" prefWidth="-1.0">
          <children>
            <Slider fx:id="volumeSlider" showTickLabels="true" showTickMarks="true" value="50.0" HBox.hgrow="ALWAYS" />
          </children>
        </HBox>
        <HBox alignment="CENTER" prefHeight="-1.0" prefWidth="-1.0">
          <children>
            <Button fx:id="playButton" disable="true" mnemonicParsing="false" text="Play" />
          </children>
          <padding>
            <Insets bottom="5.0" left="5.0" right="5.0" top="5.0" fx:id="x1" />
          </padding>
        </HBox>
      </children>
    </VBox>
  </bottom>
  <center>
    <TitledPane fx:id="titledPane" animated="false" maxHeight="1.7976931348623157E308" text="Files">
      <BorderPane>
        <center>
          <ListView fx:id="musicList" prefHeight="200.0" prefWidth="200.0" />
        </center>
        <top>
          <HBox alignment="CENTER" padding="$x1" prefHeight="-1.0" prefWidth="-1.0" spacing="15.0" BorderPane.alignment="CENTER">
            <children>
              <Label fx:id="currentDirectoryLabel" text="" wrapText="true" HBox.hgrow="NEVER" />
              <Button mnemonicParsing="false" onAction="#chooseDirectory" text="Browse..." />
            </children>
          </HBox>
        </top>
      </BorderPane>
    </TitledPane>
  </center>
</BorderPane>

MP3Player.java

package mp3player;
	
import javafx.application.Application;
import javafx.stage.Stage;
import javafx.scene.Scene;
import javafx.scene.layout.BorderPane;
import javafx.fxml.FXMLLoader;


public class MP3Player extends Application {
	@Override
	public void start(Stage primaryStage) {
		try {
			FXMLLoader loader = new FXMLLoader(getClass().getResource("MP3Player.fxml"));
			BorderPane root = loader.load();
			MP3PlayerController controller = loader.getController();
			Scene scene = new Scene(root, 300, controller.getWindowHeight());
			primaryStage.setScene(scene);
			primaryStage.show();
		} catch(Exception e) {
			e.printStackTrace();
		}
	}
	
	public static void main(String[] args) {
		launch(args);
	}
}

MP3PlayerController.java

package mp3player;

import java.io.File;
import java.io.IOException;
import java.nio.file.FileSystems;
import java.nio.file.Path;
import java.nio.file.PathMatcher;
import java.nio.file.Paths;
import java.nio.file.Files;
import java.util.Arrays;
import java.util.stream.Collectors;
import java.util.stream.Stream;

import org.fxmisc.easybind.EasyBind;
import org.fxmisc.easybind.monadic.MonadicBinding;
import org.fxmisc.easybind.monadic.MonadicObservableValue;

import javafx.animation.KeyFrame;
import javafx.animation.KeyValue;
import javafx.animation.Timeline;
import javafx.beans.binding.Bindings;
import javafx.beans.binding.BooleanBinding;
import javafx.beans.property.ObjectProperty;
import javafx.beans.property.SimpleObjectProperty;
import javafx.collections.FXCollections;
import javafx.collections.ObservableList;
import javafx.event.ActionEvent;
import javafx.event.EventHandler;
import javafx.fxml.FXML;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.control.Label;
import javafx.scene.control.ListCell;
import javafx.scene.control.ListView;
import javafx.scene.control.ProgressBar;
import javafx.scene.control.Slider;
import javafx.scene.control.TitledPane;
import javafx.scene.media.Media;
import javafx.scene.media.MediaPlayer;
import javafx.scene.media.MediaPlayer.Status;
import javafx.stage.DirectoryChooser;
import javafx.stage.Window;
import javafx.util.Duration;

public class MP3PlayerController {
	
	private static final Duration FADE_DURATION = Duration.seconds(2.5);
	
	@FXML
	private Slider volumeSlider ;
	@FXML
	private Button playButton ;
	@FXML
	private ListView<Path> musicList ;
	@FXML
	private Label currentDirectoryLabel ;
	@FXML
	private ProgressBar currentTimeIndicator ;
	@FXML
	private Label timeLabel ;
	
	@FXML
	private TitledPane titledPane ;
	
	private DirectoryChooser directoryChooser  = new DirectoryChooser();

	private MonadicObservableValue<MediaPlayer> player ;
	
	private final ObjectProperty<Path> currentDirectory = new SimpleObjectProperty<>();
		
	// These will be used to keep track of the last known window height
	// independently for when the titled pane is collapsed or expanded.
	// When the expanded state changes, we will restore the window height to the
	// last known value for the new state.
	private double expandedWindowHeight = 400 ;
	private double collapsedWindowHeight = 200 ;
	
	double getExpandedWindowHeight() {
		return expandedWindowHeight ;
	}
	
	double getCollapsedWindowHeight() {
		return collapsedWindowHeight ;
	}
	
	double getWindowHeight() {
		if (titledPane == null || titledPane.isExpanded()) {
			return expandedWindowHeight ;
		} else {
			return collapsedWindowHeight;
		}
	}
	
	public void initialize() {
		
		player = createObservablePlayer();
		
		configureCurrentDirectory();
		
		// Cross-fade when changing players
		player.addListener((obs, oldPlayer, newPlayer) -> {
			fadeOut(oldPlayer);
			fadeIn(newPlayer);
		}); 
		
		configureSongTimeDisplay();
		
		
		// Automatically invoke stop and reset play location at end of media:
		player.addListener((obs, oldPlayer, newPlayer) -> {
			if (oldPlayer != null) {
				oldPlayer.setOnEndOfMedia(null);
			}
			if (newPlayer != null) {
				newPlayer.setOnEndOfMedia(() -> {
					newPlayer.stop();
					newPlayer.seek(newPlayer.getStartTime());
				});
				newPlayer.seek(newPlayer.getStartTime());
			}
		});
		
		
		// Bind text and disabled property of Play/Pause button, and register action listeners:
		configurePlayButton();
				
		configureMusicList();
		
		resizeOnTitledPaneExpand();
		
	}

	private void configureCurrentDirectory() {
		
		// Possible directories under user's home directory for startup
		
		Stream<String> searchPaths = Arrays.stream(new String[]{
				"Music/iTunes/iTunes Music",
				"My Music/iTunes/iTunes Music",
				"Music/iTunes",
				"My Music/iTunes",
				"Music",
				"My Music",
				""
		});
		
		// Configure current directory and label:

		final String userHome = System.getProperty("user.home");
		Path file = searchPaths.map(name -> Paths.get(userHome, name))
				.filter(Files::exists)
				.findFirst()
				.orElse(Paths.get(userHome)); // should not happen...
		
		currentDirectory.set(file);
		currentDirectoryLabel.textProperty().bind(EasyBind.map(currentDirectory, Path::getFileName).map(Path::toString).map("Directory: "::concat));
		directoryChooser.initialDirectoryProperty().bind(EasyBind.map(currentDirectory, Path::toFile));
	}

	private void configureMusicList() {
		
		// Bind ListView's items to list of appropriate files in current directory
		
		musicList.itemsProperty().bind(EasyBind.map(currentDirectory, this::listFiles)
				.orElse(FXCollections.emptyObservableList()));
				
		// custom cell factory to display only the file name, without the extension:
		musicList.setCellFactory(lv -> {
			// create a default cell
			ListCell<Path> cell = new ListCell<Path>();
			// bind the text property
			// the monadic takes care of null cell items and maps to null text
			cell.textProperty().bind(EasyBind.monadic(cell.itemProperty())
					.map(Path::getFileName)
					.map(Path::toString)
					.map(name -> name.substring(0, name.lastIndexOf('.'))));				
			return cell ;
		});
	}
	
	// List of supported files in the given directory:
	private ObservableList<Path> listFiles(Path dir) {
		
		PathMatcher matcher = FileSystems.getDefault().getPathMatcher("glob:**.{mp3,m4a,aiff,aif,wav}");
		try {
			return Files.list(dir)
					.filter(matcher::matches)
					.collect(Collectors.toCollection(FXCollections::<Path>observableArrayList));
		} catch (IOException exc) {
			return FXCollections.observableArrayList();
		}

	}
	
	private MonadicObservableValue<MediaPlayer> createObservablePlayer() {
		// Bind player to selected item:
		return EasyBind.monadic(musicList.getSelectionModel().selectedItemProperty())
				.map(this::createPlayer);
	}
	
	private MediaPlayer createPlayer(Path file) {
		try {
			return new MediaPlayer(new Media(file.toUri().toURL().toExternalForm()));
		} catch (Exception exc) {
			exc.printStackTrace();
			return null ;
		}
	}
	
	private void configureSongTimeDisplay() {
		// current position of play head for current song, in seconds, or 0 if no song:
		final MonadicBinding<Double> currentTimeBinding = 
				player.flatMap(MediaPlayer::currentTimeProperty)
				.map(Duration::toSeconds).orElse(0d);
		
		// total duration of current song, in seconds, or 0 if no song:
		final MonadicBinding<Double> totalTimeBinding = 
				player.flatMap(player -> player.getMedia().durationProperty())
				.map(Duration::toSeconds).orElse(0d);
		
		// bind text of label to formatted play head position and total time:
		timeLabel.textProperty().bind(EasyBind.combine(
				currentTimeBinding,
				totalTimeBinding,
				this::formatTimeLabel));
		
		// bind progress bar to current play head position as proportion of total time:
		currentTimeIndicator.progressProperty().bind(EasyBind.combine(
				currentTimeBinding, 
				totalTimeBinding, 
				(currentTime, totalTime) -> currentTime / totalTime));
	}

	private String formatTimeLabel(double currentTime, double totalTime) {
		return String.format("%s / %s", formatTime(currentTime), formatTime(totalTime));
	}
	
	private String formatTime(double time) {
		int mins = (int)time / 60;
		int secs = (int)time % 60 ;
		return String.format("%d:%02d", mins, secs);
	}
	
	private void configurePlayButton() {
		
		// Disable button if there's no player
		playButton.disableProperty().bind(Bindings.isNull(player));
		
		BooleanBinding playing = Bindings.equal(player.flatMap(MediaPlayer::statusProperty), Status.PLAYING);
		
		// Bind play button text to "Pause" if there's a player with PLAYING status, "Play" otherwise
		playButton.textProperty().bind(Bindings.when(playing).then("Pause").otherwise("Play"));

		// Bind onAction to pause() if playing and play() otherwise:

		playButton.onActionProperty().bind(Bindings.when(playing)
				.<EventHandler<ActionEvent>>then(event -> player.get().pause())
				.otherwise(event -> player.get().play()));
	}
	
	private void fadeOut(MediaPlayer mp) {
		if (mp != null) {
			mp.volumeProperty().unbind();
			Timeline fadeOut = new Timeline(new KeyFrame(FADE_DURATION, new KeyValue(mp.volumeProperty(), 0)));
			fadeOut.setOnFinished(event -> mp.stop());
			fadeOut.play();
		}
	}
	
	private void fadeIn(MediaPlayer mp) {
		if (mp != null) {
			mp.volumeProperty().unbind();
			Timeline fadeIn = new Timeline(new KeyFrame(Duration.ZERO, new KeyValue(mp.volumeProperty(), 0)),
					new KeyFrame(FADE_DURATION, new KeyValue(mp.volumeProperty(), volumeSlider.getValue() / 100 )));
			fadeIn.setOnFinished(event -> mp.volumeProperty().bind(volumeSlider.valueProperty().divide(100)));
			mp.play();
			fadeIn.play();
		}
	}


	private void resizeOnTitledPaneExpand() {
		
		// keep track of current window height (monadic binding will be empty if no scene or window yet):
		MonadicBinding<Number> windowHeight = EasyBind.select(titledPane.sceneProperty())
				.select(Scene::windowProperty)
				.selectObject(Window::heightProperty);

		// track last height independently for when the titled pane is collapsed or expanded:
		windowHeight.addListener((obs, oldValue, newValue) -> {
			if (titledPane.isExpanded()) {
				expandedWindowHeight = newValue.doubleValue();
			} else {
				collapsedWindowHeight = newValue.doubleValue();
			}
		});
		
		// restore window height to last known value for given expanded state
		// when titled pane is expanded or collapsed:
		titledPane.expandedProperty().addListener((obs, wasExpanded, isExpanded) -> {
			Window stage = titledPane.getScene().getWindow() ;
			if (isExpanded) {
				stage.setHeight(expandedWindowHeight);
			} else {
				stage.setHeight(collapsedWindowHeight);
			}
		});
	}
	
	// handler for button to change directory:
	@FXML
	private void chooseDirectory() {
		File dir = directoryChooser.showDialog(musicList.getScene().getWindow());
		if (dir != null) {
			currentDirectory.set(dir.toPath());
		}
	}
}

Nice use of EasyBind!

For this particular case though

player.map(p -> false).orElse(true)

JavaFX already has a baked-in solution

Bindings.isNull(player)

Ah, yes, that is better in that case. I think I was just trying to see if I could eliminate all of the Bindings calls (which I understand is not the point, but it was a fun exercise).

The other statements in the same method feel a bit clunky too. Perhaps I should do

BooleanBinding playing = Bindings.equal(player.flatMap(MediaPlayer::statusProperty), Status.PLAYING);

and then use a

Bindings.when(...).then(...).otherwise(...)

construct