I've had this problem a lot (can be found
http://discussions.apple.com/thread.jspa?threadID=1847650 there as well, but I can't reply) and I think it's just a bug in the system. It's pretty easy to fix though without having to create new accounts or reinstall the os or do anything crazy. Here's the trick that always seems to work for me:
Open up your Applicaitons folder, and find the "Utilities" folder with in it. In there is a utility called the Audio MIDI with a default icon of a keyboard. Open that puppy up, and at the top you'll see two tabs. You want the one on the left, the one that gives you sixteen thousand different options for selecting inputs and outputs. The screen should be divided into two columns, with a bunch of drop down select menus to choose internal speakers, or the on-board mic, and what not. There's also a selection menu for the sample frequency, which reads in KHz, something like "48,000 or 44100 or 32000" etc. All you have to do is change that number to a different frequency. Restart. Hopefully you've unlocked your sound and volume control.
If you need to, you can go back into that utility and change it back to default, or sometimes I have to pull the three (M, 1, and 2) sliders from the 0 left to Full right manually, but it always seems to work for me. Hope it helps you!
