Ok, back to the content that I originally started to post before I realized that my audio wasn't working on hardware. Which is: I think I'm pretty much done! (again...didn't I say that before?)
Turns out (for those of you interested in the details) that something somewhere was sending some dummy messages over the IPC FIFO (the mechanism by which the arm9 and arm7 processor communicate). Because the only messages that I send over it were instructions to play songs, I assumed any messages received were instructions to play songs, and it was trying to play a nonexistent song. Better sanity checking on the commands coming through IPC fixed that.
So now I've made a test build. I'll sanity check it over the next day or two, then send it out to anyone interested in helping test. Then call this thing done!