The only things I can think of:
Do you have voice chat enabled inside the game? That might cause an interference if you do.
Do you have a hotkey enabled (check the Vent setup) and if so, does it conflict with a game control? Stock hotkey is left control, which is a game control. I reset mine to Q and Vent works just fine. I do have to adjust the volumes inside the game so I can hear what's being said on Vent.
'09 Kawi Versys