A problem with Headphones on my iMac

I can't get my iMac to push sound through my Logitech usb headphones. This is on Windows through bootcamp obviously. The weird part is that I don't have any problem with ventrillo, but when I play RIFT I can't get the game sound to come through the headphones. I don't know if this is a Windows problem or an iMac problem. Any help would be appreciated.