So Apple introduced access to Iphone’s iPod library (iTunes) with new 3.0 iPhone OS. Here’s nice introduction how to use it. You can use Media item picker or media queries to select audio-based media items from iPhones music library to be played using Music Player. However the returned media items can only be played using MPMusicPlayerController, which provides only high level access to audio files.

That means that you can play them, read metadata, etc. but you cannot analyze or alter the media files in any way. So pretty much only thing you can do with it is to make a different UI for iPod. It would be so much more useful if you could get the raw data, which would enable FFT, and other audio processing.