Writing to the dsp device
Sound output seems to be generally done with SDL, or mikmod libraries, but it could be done without any sofisticated libraries. The standard way under Linux seems to be to open the dev/dsp device and play with it. Unlike other devices, dsp must be open either in readonly or writeonly, but not both. This can be explained by the fact that opening for reading will link to te audio input (microphone, if the GP2X had one) and opening in write will deal to the speakers.
This is indeed a stub, any contribution is welcome, meanwhile, see http://www.oreilly.de/catalog/multilinux/excerpt/ch14-05.htm and dzz demo tutorial