I'll give a tip on how I solved this problem, which may be a little different.
I was building an app where I wanted the user to be able to preview an MP3 clip, even if they had the phone on silent.
The way I went about this was first, importing the Audiobox framework <AVFoundation/AVFoundation.h>
at the start of your file and then later in my code before I initialized my AVAudioPlayer I did:
[[AVAudioSession sharedInstance]
setCategory: AVAudioSessionCategoryPlayback
error: nil];
Worked like a charm. It may not be what the OP needed, however.