Internal mic not working on 2008 iMac running 10.7.5
My guess is that they accidentally disconnected the mic or failed to reconnect the mic when they operated on it.

MacFixIt has a guide if you want to try to take it on yourself.  They rate is as "Moderate" difficulty.

Be sure to double check that I got the right model for your iMac.  

Otherwise it's back to Apple it goes...