I was using this answer for how to fade out music player volume in my app, but in iOS 5, this leads to the on-screen HUD for volume appearing to the user, as if they had pressed the volume buttons on the side of their device. Does anyone know a workaround that doesn't display the HUD?
kurtzmarc
I found a solution here: applicationMusicPlayer volume notification
It simply directs you to add an MPVolumeView to your view. For some reason, having that in the view hierarchy prevents the volume HUD from appearing. Works for me, hopefully for you too.
来源：https://stackoverflow.com/questions/7797726/fade-out-playback-volume-on-mpmusicplayercontroller-on-ios-5