First, after reading a technote at Apple (http://docs.info.apple.com/article.html?artnum=93218), we zeroed in our trouble-shooting to my buddy's Airport network. That appeared to be the chief source of our problems. First, he went back to an Ethernet connection and our silent video conferencing troubles were over, consistantly. Then we tried with him going back to Airport but both of us limited our bandwidth to 500 kps. Could be coincidence, but we were able to do video conf with sound.
I have used both my camera mic and an external mic I still had shrink wrapped from my old 8500 (I currently use a dual 500 G4 PowerMac). When we had no audio, that never seemed to make a differnce, though, only a few times, my iChat AV would crash as I switched from one mic to another using the pulldown in the "video" portion of the prefrences.
My broadband service is SpeakEasy (Covad) DSL and my buddy has Earthlink via Time-Warner cable. He's a relatively new Mac user and I had to walk him through how to effectively use the Locations in the Network prefs so he didn't have to spend so much time reconfiguring when he switched from Ethernet to Aiport. That worked well, though his cable gateway is kinda funky and seems to require a reset when he switches from wire to wireless. Also, he does not actually use the Airport base. He simply has cards in both an iMac and an iBook and does a software base. Surprisingly, that does seem to work most of the time.
Summary, when my buddy uses Ethernet we have no silent video problems. If he uses Aiport and a 500kps limit, we seem to be able to both audio conf and video conf wih sound most of the time.