I know you sound like you've tried this, but last night my Mac did the same thing. No sound. I plug in an adaptor in my head-phone jack to some RCA wires that runs though my walls and into the living room to hook up to my receiver so I can play tunes in my kitchen and around the house through built-in speakers.
Sometimes, I have to plug in the headphone jack and pull it out a few times at the back of my Mac before the sound works again. Seems like something gets stuck in there and thinks there are headphones plugged in when there isn't.
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