Your First Mac App -- updateUserInterface reference issue

AppDelegate.h

#import <Cocoa/Cocoa.h>

@class Track;

@interface AppDelegate : NSObject <NSApplicationDelegate>

@property (assign) IBOutlet NSWindow *window;
@property (weak) IBOutlet NSTextField *textField;
@property (weak) IBOutlet NSSlider *slider;

@property (strong) Track *track;

- (IBAction)takeFloatValueForVolumeFrom:(id)sender;
- (IBAction)mute:(id)sender;
- (void)updateUserInterface;
@end

AppDelegate.m

#import "AppDelegate.h"
#import "Track.h"

@implementation AppDelegate

@synthesize track;
@synthesize window = _window;
@synthesize textField = _textField;
@synthesize slider = _slider;

- (void)applicationDidFinishLaunching:(NSNotification *)aNotification
{
    Track *aTrack = [[Track alloc] init];
    [self setTrack:aTrack];
}

- (IBAction)takeFloatValueForVolumeFrom:(id)sender {
    float newValue = [sender floatValue];
    [self.track setVolume:newValue];
    [self updateUserInteface];
}

- (IBAction)mute:(id)sender {
    [self.track setVolume:0.0];
    [self updateUserInterface];
}

- (void)updateUserInterface {
    float volume = [self.track volume];
    [self.textField setFloatValue:volume];
    [self.slider setFloatValue:volume];
}

@end