获取手机左边音量＋ －按键的事件方法或私有api

 MPMusicPlayerController *ipodMusicPlayer = [MPMusicPlayerController iPodMusicPlayer];
    [ipodMusicPlayer beginGeneratingPlaybackNotifications];
    [[NSNotificationCenter defaultCenter] addObserver:self 
                                             selector:@selector(trackTheDeviceVolume:) 
                                                 name:MPMusicPlayerControllerVolumeDidChangeNotification 
                                               object:nil];