I've been working in Core Audio on iOS recently as it got some major upgrades with iOS5 which make it a lot easier to play MIDI based music on the iPhone and iPad. I'm hoping to write some tutorials to bring together the things I've learnt in the hope it will help others. As anyone who has checked out Core Audio will know the documentation is good in some parts but woefully deficient if not completely non-existant in others.
The new(to iOS) MusicPlayer class makes loading, generating and playing sequenced music through a Core Audio Graph much easier than it used to be. Event timing is dealt with by the MusicPlayer and is has some good methods for rearrange and iterate through event data.
Combine this with the new AUSampler and you can pretty much get a music app up and running with minimal hand coding of Audio Units to deal with MIDI and audio generation.