I have a 4750, so I can't really give any first-hand experience help, but try this:
Go to the sound manager thing, go File>Properties, Recording, check EVERY box. Hit ok, now try checking the SELECT next to the first option, see if your mic works, try selecting the next one, until you try every single option. Maybe your internal one is under a different name, such as Aux or something..
If you have a microphone plugged in to the MIC port, it will disable the internal mic, but I'm sure you knew that. Again, make sure that in Playback nothing is muted, and in Recording your Mic is selected.
Maybe re-install your sound drivers?