I've written a multi-track playback mechanism, No GUI for it or anything at the moment, but everything streams very nicely. It supports a block-based audio data approach, like on most digital recorders, so you'll be able to drag around music sections and the like. I'll be adding recording capability to it at some point, and eventually get round to doing a GUI for it. I haven't managed to do any QT stuff yet though.
I was thinking of turning it into a tracker, but that would mean adding sample rate conversion which is not necessary for multi-track recording. Also, everything streams from disk at the moment and a memory-based approach is better for trackers. I'll think about it.
Hmmm....