I had a similar problem. In my case the drivers were fine, windows 7 was detecting the in built microphone, but I wasnt picking up any sound, laptop about a year old.
I think a lot of these microphone problems people have with their laptops is with the wire connecting the mic to the mothetboard, running through the hinge. I noticed that the microphone started working if the laptop screen was open less than about 75 degrees, but one open to 90 or more it stopped.
Something had come loose or split but this might be the same trouble you're, and a lot of others, are having.