OK, so discovered that my microphone wasn't working. What's the diagnostic procedure. Well it comes down to a bunch of simple things to try:
- Go to System Preference/Sound make make sure the internal microphone is selected and not muted
- Reboot the machine and hold down the four keys Option-Command-P-R to reset the PRAM configuration information
- Go to Applications/Utilities/MIDI Setup and make sure that input isn't muted
- If all these don't work, then you might have a deep operating system problem, so boot into a completely different operating system. Easiest way to do this is to install a bootable copy of Mac OS X onto a USB Flash Drive and boot to it and see if you find sound
If none of these works (they didn't for me), you probably have a hardware problem. I took to the Genius Bar and they replaced a $12 left speaker and microphone assembly. Said sometimes the cable would just get loose.