Well, that was quick xD

Long story short, I got bored a few days ago, so I wrote out the whole music engine in assembler, got bored again, and ported it to the DS just cos I can =D

Sonic AS has 16 channels polyphony for the DS, and variable polyphony on the GBA. It has support for variable music players, and the DS version can handle PCM8/16 and ADPCM data, as it uses the hardware channels. The GBA version can also have either nearest neighbour interpolation (that is, no interpolation; this is fast), or linear interpolation, with .23 fixed point accuracy, which can be toggled by changing the variables in sasInternal.inc.
Since it's written in all assembler, the memory footprint is rather low.

All processing is done on the ARM7 side, with the ARM9 simply signaling to play/stop, etc.

As per usual, it comes with all sources, assets, etc., and yeah.