Mac OS X forum: OS 10.6- Missing driver for internal speakers.

Try resetting the system's PRAM, by rebooting and immediately holding down the options-comman-P-R keys all at once. The system will make the boot chimes, and then reboot and repeatedly do so as long as the keys are held. Let it cycle like this a few times and then release the keys and allow it to boot normally.

I had the same problem on the new iMac, seeing only the headphones output for Sound in preferences, even though my iTunes played normally through the external speakers that are plugged in. So, I tried resetting PRAM with the wireless keyboard that came with the iMac and it did not seem to work -- I got only the normal reboot and no chimes (as I have gotten over the years on the other computers).

So, I got the plug-in keyboard from the old iMac and used that and got three sets of chimes before I allowed it to boot normally.

AND NO DIFFERENCE. Still got the same lack of output choices under Sound Prefs.

Going to do this once more and see if that corrects it, then go out and search the userbase for any other ideas.

I wanted to use Skype and the video chat and got the video to work fine, just no audio.

Guess I will also hunt up a set of compatible headphones and try that...

Square Wheels are everywhere -- things work, just not smoothly...

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If this is a newer iMac, then try looking at the actual headphone jack. Is there a red light coming out from it? If yes, then it's stuck in digital output mode. SOMETIMES you can fix that by repeatedly plugging something in and out of the jack. Other times it can be fixed if you reseat the cable that connects the headphone and mic jack to the logic board, but on an iMac you don't want to be messing with that unless you know what you're doing. If that doesn't work, then the logic board's audio chip is basically broken and you need a new logic board.

There are also posts about on the Internet about there being a stuck pin inside the jack that you can fix, but you probably want to be careful about that. Those two audio jacks are not part of the logic board on a unibody iMac (any model where the glass goes all the way to the edge of the system), so if you go digging around in there and break something, even if you get the logic board replaced, you'll still be screwed. And while someone may not look too closely before replacing the logic board, if that doesn't fix it, the next place to look would be those jacks, and if the tech sees evidence of you having dug around n there, you just voided any warranty you may have had.