I already have the source code to that plugin and an Xbox Adpcm decoder + source written by the same author, plus I started working on Xbox Adpcm support yesterday. Trust me, it's VERY helpful. As an added bonus, I'm also working on XACT support for Unreal. Xbox Audio is a real challenge (for me), so don't get too excited. Last time I tried adding 3D sound support, and it broke Turok! But if it does work properly, then it will be in the next release.
I was going to do it before but I forgot until recently...
More on the Xbox Adpcm codec thing... it's been a very minor thing on Caustik's todo list (as with most sound related things). The decoder was written by Luigi Auriemma, and lucky enough for us, his source code is released under the GPL just like Cxbx. The biggest trick is to decode the Xbox Adpcm data into regular Pcm data. IIRC, DirectSound buffers (for the PC) only playback audio data in Pcm format. This shouldn't be hard.
Good:
- Saves us time trying to figure out for format ourselves.

- Should play both stereo and mono sounds compressed with this codec (so it's not limited to a specific file format).
Bad:
- No idea how streaming buffers would be effected (may get crappy output or crashes). Additional hacks may be required.
- Requires more overhead to decode.
- Cxbx's audio output is far from acceptable in many areas; meaning there's more sound related bugs than you can shake a stick at to begin with. An option to completely disable DirectSound may be a good idea (some games like Quantum Redshift may need). As far as sound goes, Xeon beats Cxbx by a long shot!
Thanks for asking though, such input is helpful. Which reminds me, I need to send the link to shadow_tj because there's a delphi port and he could really use it!