Sound keeps playing on Sprite Kit game

I used this code to loop a song in my Sprite Kit game but when I go out of the app/game the song keeps playing and I don't know how to stop the song when you get off the game:

    url = [NSURL fileURLWithPath:[[NSBundle mainBundle] pathForResource:@"deep-blue-sea" ofType:@"wav"]];
    _sound = [[AVAudioPlayer alloc] initWithContentsOfURL:url error:nil];
    _sound.delegate = self;
    _sound.numberOfLoops = -1;
    [_sound play];

Please can someone help me how to do it?