AVAudioPlayer doesn't play sound when MPMusicPlayerController is playing on iOS

Question

I have an app that plays various musical sounds using AVAudioPlayer. It lets people select a song from their library and plays it with MPMusicPlayerController. It used to work that they could jam with it, but now when MPMusicPlayerController is playing it doesn't play the sound using AVAudioPlayer. It even doesn't play the sound once MPMusicPlayerController has stopped. I've tested it on 13.4.1 and 13.6. What changed to make it stop working and what can I do to fix it?

It plays the sound like this:

AVAudioPlayer *player = [[AVAudioPlayer alloc] initWithContentsOfURL:url error:&err];
[player prepareToPlay];
[player setVolume: 1.0];
[player play];

For the music, it gets a MPMediaItemCollection using MPMediaPickerController, then plays it like this:

- (void) playMusic: (MPMediaItemCollection *) collection {
    [[AVAudioSession sharedInstance] setCategory: AVAudioSessionCategoryAmbient error: nil];
    if (musicPlayer == nil) {
        musicPlayer = [MPMusicPlayerController applicationMusicPlayer];
        [musicPlayer setShuffleMode: MPMusicShuffleModeOff];
        [musicPlayer setRepeatMode: MPMusicRepeatModeNone];
    }
    [musicPlayer setQueueWithItemCollection:collection];
    [musicPlayer play];
}

Answer 1

Feeling stupid. The phones were set to silent mode. For some reason, it can play the sounds or the music on silent, but it can't play them both together.