alansvits

Put the code in the class that actually has the data. Have the class register for the UIApplicationDidEnterBackgroundNotification notification.

// Put this in the `init` method
[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(backgrounding) name:UIApplicationDidEnterBackgroundNotification object:nil];

// The method that gets called
- (void)backgrounding {
    // save the data
}

alansvits

import AVFoundation
    
    var audioPlayer: AVAudioPlayer!

    let soundURL = Bundle.main.url(forResource: "soundName", withExtension: "soundFormat")

    do {
        audioPlayer = try AVAudioPlayer(contentsOf: soundURL!)
    } catch {
        print(error)