app内播放音视频，不影响后台音乐播放（添加此句才可以将控制权交给app，否则后面的播放／停止后台音乐无效）
[[AVAudioSession sharedInstance] setCategory:AVAudioSessionCategoryAmbient withOptions:AVAudioSessionCategoryOptionMixWithOthers|AVAudioSessionCategoryOptionDefaultToSpeaker error:nil];
AVAudioSessionCategoryOptionMixWithOthers 是否多个音频混合在一起
AVAudioSessionCategoryOptionDefaultToSpeaker 使用默认扬声器
暂停背景音乐
AVAudioSession *session = [AVAudioSession sharedInstance];
[session setCategory:AVAudioSessionCategoryPlayback error:error];
[session setActive:YES error:nil];
继续播放背景音乐
[[AVAudioSession sharedInstance] setActive:NO withOptions:AVAudioSessionSetActiveOptionNotifyOthersOnDeactivation error:nil];
how to don't stop music when the app start launching
https://stackoverflow.com/questions/18762544/ios-how-to-dont-stop-music-when-the-app-start-launching
继续播放或暂时其他app的背景音乐
https://blog.csdn.net/u013538542/article/details/69263777
iOS AudioSession详解 Category选择 听筒扬声器切换
https://blog.csdn.net/xy5811/article/details/8563137
app内播放音视频，不影响后台音乐播放
https://blog.csdn.net/sir_coding/article/details/61623608
