I have no idea what your title has to do with the thread
But anyway

start > settings > control panel > sounds and audio devices

Go through the tabs,
probably the Audio tab is what you'll be looking at.
Make sure everything looks right.

DId this happen just randomly?
Did you do anything recently?
Well shoot.
I know less about Mac then I do about girls, so I really can't help you with that, sorry.
Hopefully someone else who uses a mac will come around.
I think I know whats wrong with your sound device...

Tits on a whale!
*runs away*