It was brought up today earlier in chat. While I don't know how practical it would be to code, Im not sure it would need to be done. What would yall think of something like Mumble, its like IRC for voice basically.
I looked into Ventrillo and it seems to have a higher overhead. I also installed mumble and it seems to have decent options for how to activate the mic (always on, volume activated with adjustable threshold, push to talk). The setup for me actually had a lot of weird beeping / squeaks, but jumping into a couple of random voice servers people said they heard me fine and the post install tests seemed to go OK.
We would need some type of room/server for it though, so I suppose either the game server or a personal PC would have to act as the server (which there is an option to setup files to install as server). The install is roughly 64MB.. incredibly small.
I found it here
http://www.makeuseof.com/tag/3-free-voice-chat-programs-pc-gamers/http://sourceforge.net/projects/mumble/?source=dlp