hey lwfitz. I think I remember something about this in the past. I also think it came down to being something to do with Catalyst. Wish I could be more helpful. Also, I may be talking out of the other side of my mouth! It may have just been a bad dream. (I personally have never utilized the HDMI hardware on my lappy since starting all this excitement!) I've also been under the impression that the HDMI audio is controlled via the video card and only works if plugged in via an HDMI cable. However, I'm not sure of that either.
So in other words, just forget I made this post!!!