A new version (0.05) of the OpenXDK has just been released.
The feature that is of most interest is audio support. Currently, only 16-bit stereo 44kHz samples are supported, but auto-conversion for other types will be supported soon. The SDL_Audio layer will attempt to convert to something playable, but the conversion is still experimental.
In addition to this, OpenXDK also supports SDL_File, SDL_Timer, SDL_Thread, SDL_Mutex and SDL_Semaphore.
This release also contains a couple of minor bugfixes to the newlib/libc layer.
A huge thank you to Andy Green (for writing the original Cromwell sound layer and answering my ridiculously naive audio questions) and also to the guys on the comp.lib.sdl newsgroup for their patience and advice.