I just need an IO Unit with a processing callback with a simple play and stop feature.
Apple has this giant MixerHost demo with thousands of lines of code just to play two mixed audio files.
It seems 99% of that code is boilerplate to set things up.
Maybe there is a open source framework which deals with this boilerplate such that you can just set up your Audio Session and start constructing a simple processing graph with an IO Unit?
Take a look at Novocaine, an analgesic for high-performance audio on the iPhone, iPad and Mac OS X. Really fast audio in iOS and Mac OS X using Audio Units is hard, and will leave you scarred and bloody. What used to take days can now be done with just a few lines of code.