I can't talk in game on Xbox live. I can talk in the lobby but not in game. Is there something I'm not doing??

What game are you playing and on what platform i.e. Xbox or Xbox360??

If GR or GRIT on Xbox the game has a push to talk system, you need to push and hold the white button on the Xbox pad to talk.

If playing GR or GRIT on Xbox360 the voice communication does not work on Xbox360 Live.

I hope that helps, if not please provide more information on the game and platform and hopefully we can sort it out