I just got an email indicating that the powers that be have just published the Ducking (automatic volume adjustment for communications applications) whitepaper that I mentioned during my PDC Talk. It's the whitepaper entitled "Stream Attenuation in Windows 7". This is a preliminary version of the final ducking documentation and should be good enough to help anyone wanting to work with the ducking feature get started. It also covers some of the subtleties that I wasn’t able to cover in my talk.
EDIT: Added which paper it was and a definition of "Ducking".