voice for multiplayer

I tried using mutiplayer last night and it was pretty good. I have a headset that I used to use for an online sim game that was done through TeamSpeak. When I went into a 'room' last night I could hear the ATC guy but couldn't talk to him as my mic doesn't have a talk button. I use to map it through T/S onto a button on my keyboard. Is there a default button on the keyboard that you use? Any help would be appreciated