Mumble problem (mic problem) ?

[Killuah15]

SORRY I'm going to post this problem in the PR:BF2 support section (I don't know how to delete a thread, I'm sorry)

Hey there, recently my cousin wanted to play some Project Reality with me, but he can't talk to me over Mumble.
He said, that in the options menu, the mic works and records his voice, but i can't hear him In-game!
Sometimes i can hear a kind of a "knack" sound (but that could also be, because of the "radio-effect" i guess...)!
We tried nearly everything, but haven't found a solution yet, i just hope that you guys can help us, we really want to play PR again, you know?
(btw. i hope I'm in the right place with this topic, I'm sorry If i'm not!)