Can you speak for a while while in-game and then it stops, or does it just stop as soon as you are in game? What happens when you alt-tab and try talking? Does that work?
I would play around under Settings -> Options -> Capture, the Capture Mode and Capture Device settings. Try not having using the "default device", and use the device that seems to work for you when outside the game. I'm wondering if it resets that to something else.
Good luck!
