Mic Problems.. Only in Call of Duty 2

[Psy]

Ok here's a toughy!..

My brother has a microphone which he uses to converse on teamspeak.
The mic works fine while just on ts, or when he plays all games.. All the time works fine.. EXCEPT.. when playing Call of Duty 2.. When playing Call of Duty 4, it works.. and all other games.

Any ideas why it may be?

He's on vista and the mic is brand new. All be it it did the same with an older mic so its not that.

Many Thanks,

Psy