Audio on the GBA is mostly GB audio plus two 8-bit streams channels, capable of software mixing (which is equivalent to what the PC SoundBlaster would have done with samples-based music). On a DS, you would just have to use the GBA mode to get access to that... on a DSi, afaik, you don't have that anymore, but you do have 16 independent samples channel, so it's merely a matter of using gameboy-compatible instruments to make it sound chippy. I'd suggest nitro tracker for that. (tobw.net)
HTH.