在应用程序中，停止正在后台播放的音乐

如果在播放则停止，如没有播放则播放

MPMusicPlayerController *musicPlayer = [MPMusicPlayerController iPodMusicPlayer];
    MPMusicPlaybackState playbackState = musicPlayer.playbackState;
    if (playbackState == MPMusicPlaybackStateStopped || playbackState == MPMusicPlaybackStatePaused) {
        [musicPlayer play];
    } else if (playbackState == MPMusicPlaybackStatePlaying) {
        [musicPlayer pause];
    }