This might seem weird but whenever I go on MW3 it disables my mic and it doesn't do it on any other game. does anyone else have this problem?

My friend does, and he is about to reinstall Win in order to solve the problem.

Try configuring the game's voice options. Maybe the record level is muted or the hotkey is not assigned?

The hotkey by default is "Z" and the game has a press to speak function rather than a voice detection one.

I remember changing some dvar (possibly cl_voice to 0?) to fix this some time ago; but now the issue no longer exists for some reason.