Just dropping a note that I have ported our original PC-based musicdisk to the NDS. The musicdisk features 26 songs in a mixture of streaming (.ogg) and various module (.mod/.xm) formats, with nifty visualizers running at a full 60fps. Sound quality is extremely good when using headphones or earphones (the internal speakers are OK, but not great).
I didn't use maxmod as I have my own replay code, and I had to make some custom additions to the ARM7 side, but we finally got it done.

Thanks goes to WinterMute and the libnds team for providing the low-level libraries and a libtremor port that worked flawlessly the first time! \o/