The XAUDIO2_FLUSH_BUFFERS flag was removed because we came up with (I hope) a better design - a separate FlushSourceBuffers() API that can be used to explicitly empty the source buffer queue whenever you like, rather than just when stopping a voice. Looks like we left the old flag in the header by accident, but it will be gone in the June release.
As for forums, the ones I check semi-regularly are this one and "Forums Home » XNA Game Studio Technologies » Audio/XACT". I don't know of anything else out there that's more active - if you do, please let me know :-)
Dugan Porter [MS]
Game Audio Team
Dugan Porter [MS]
Game Audio Team