Basic class to play system sound without user interaction from adb command line. Must be compiled to .dex file

Beep.java

import android.media.MediaPlayer;
import android.media.AudioManager;
import android.util.Log;

import java.io.IOException;

public class Beep {

	/**
	 * Command-line entry point.
	 *
	 * @param args The command-line arguments
	 */
	public static void main(String[] args) {
	    (new Beep()).run(args);
	}


	private void run(String[] a) {

		MediaPlayer mp = new MediaPlayer();

		try {
			mp.setDataSource(a[0]);
			mp.setAudioStreamType(AudioManager.STREAM_NOTIFICATION);
			mp.setVolume(1f,1f);
			mp.prepare();
			mp.start();
		}
		catch (IOException e) {
			Log.w("Beep", "IOException", e);
		}

		System.out.print("Playing " + mp.getDuration() + "ms");
		int i = 0;
		while(mp.isPlaying() && i < 20) {
			System.out.print(".");
			i++;
			try {
				Thread.sleep(1000);
			} 
			catch(InterruptedException e) {
				Log.w("Beep", "IOException", e);
			}
		}
		System.out.println();
		mp.stop();
		mp.release();
		System.out.println("Stopped.");

	}

}

play_beep.sh

export CLASSPATH=./Beep.dex
app_process /system/bin Beep /system/media/audio/ringtones/Beep-beep.ogg

I see it playing on Android 9, but no sound comes out.
Any idea what could be wrong?