Just reading through this, it seems that the audio portion of the HT experience is going to be noticeably compromised. While I understand what you are trying to do, I hope you have considered other alternatives, including losing the 20" of room and not enclosing the speakers.
After having enjoyed my setup over the last three years, I can't imagine stifling the audio because it is almost more important to me now than the video portion of the HT.
Have you considered smaller speakers that you can place on the floor (M22s versus M80s)? Even a set of M3s hung on the wall near the ceiling. Anything but embedding the speakers.
Alan's advice is sound (no pun intended
about killing the audio. I'm also not sure that in-wall speakers will perform any better than boxed-in speakers.
