No audio with webcam

I purchased and installed a VX-3000 webcam on a Fujitsu T4220 laptop running
Vista Business with Windows OneCare. I have no audio when I make a video
call (I can't hear the caller, she can't hear me). I spent over an hour on
the phone with MS tech support and we verified that the microphone in the
webcam works, and the speakers in my laptop are fine. I installed the same
camera on my officemate's computer (running XP) and it's working fine, so I
don't think we have firewall issues, etc. with our office connection. I
tried disabling the OneCare firewall; that didn't resolve the problem. I've
uninstalled/reinstalled Messenger (I currently have Build 8.5 1302.1018) and
the webcam. Suggestions much appreciated, I'm very frustrated at this point!