The following code gave a warning of Sending 'ViewController *const __strong' to parameter of incompatible type 'id<AVAudioPlayerDelegate>' (it is the third line in the following code): NSURL *sound0URL = [NSURL fileURLWithPath:[[NSBundle mainBundle] pathForResource:@"0" ofType:@"aiff"]]; audioPlayer0 = [[AVAudioPlayer alloc] initWithContentsOfURL:sound0URL error:nil]; [audioPlayer0 setDelegate: s