Instructions
Click "Options" after starting the game. This is indicated by a gear icon at the top-right.
Click "Sounds"; then scroll down to the "Voice Chat" section.
Click "On" next to "Voice Chat."
Click the drop-down menu, and select your webcam microphone from the list.
Click "OK."
Press "T" while in a game to use voice chat. You must have "T" held down for others to hear you.