Check the Volume control on your pc, if its set on mute microphone, that could be it...
Otherwise it could be the cam microphone, to which i would suggest a re-install.
But then again I am a techophobe, so I could be wrong on all counts...
I had bad experience with Mike carrying cams... So I only use creative webcam and separate headset/mike. Might look old fashioned, but it works great.
Dom