Search 4 this file on ur actual user search and delete it:
~/Library/Preferences/com.apple.audio.AudioMIDISetup.plist
and then restart ur com then it should work
OSX Lion and higher this file will be hidden and you will have to show hidden files 2 see and delete it! If your mic hole has a red light in it, this vid wont help, u will need 2 use a legit toothpick :P