For what it's worth, on this MacBook Pro (17-inch, Late 2011) running 10.8.2, the only thing that worked was, literally, a toothpick. Poked and prodded with the sharp end with no luck, but the dull end (it's one of those cocktail toothpics) had it working lickety split.
Reset PRAM (several times)
Insert/remove headphones (both apple and third-party, since apple's have the extra contact)
Wiggle headphones in the jack
I never saw the red light, if it matters, even though the only output device being reported was the Digital Audio Port.
I had same issue. Mine showed speaker symbol with ∅ superimposed over it.
I had to insert the jack and move around a bit to fix. Still comes and goes when I use headphones.
I would be careful moving the jack around in the speaker/headphone output. You could permanetly bend the contacts and have to have someone open her up to fix.
I also have been noticing this. I don't think it has anything to do with Mountain Lion because I have had that OS installed since August and it's just happening now 6 months later. The startup chime sounds but then I am not able to change the sound later. Not with buttons, not in the menu bar, and not in system preferences. This is a problem because I take an online courese where I need sound to watch videos. PLEASE HELP!!!!!!
The internal speakers would not work after using headsets on my Macbook MacBookPro6,2 (mid 2010).
The only option in the sound meny was digital out which the computer does not even have.
Tried rebooting, PRAM and SMC reset, no luck.
Finally solved the problem by:
1. Opening iTunes and playing music with headphones plugged in
2. remove headphones meanwhile adjusting volume
3. If it does not work first time try again. I had to do about five tries.
the part of your solution that did the trick is the patient, repeated plugging and unplugging of the headphones.
There is a mechanical problem with a tiny switch inside the audio jack, and having iTunes open, changing the volume, or any other software-related solution that people have reported as working are pure coincidences.
p.s. your MBP does have digital out. It gets activated when you insert a fiber optic plug in the usual audio-out jack. In fact, the problem that we have all experienced is that the computer "thinks" that we are using digital out and disables analog audio accordingly.
Ok I have figured out a way running on the new Mac 10.8 Mountain Loin. This only works if you have gone to System Preferences -> Sound -> Output and it says Digital Output and when you have you headphones out the Volume is not letting you turn it up or down and is Locked on Full Volume.
First Take a pair of headphones.
Then Slow pug them in while continually pressing the up volume and down volume.
Then Slowly unplug them and you will her the volume going up and down out of you external speakers
Now your Speakers are fixed
I had the same issue.
I tried SMC Reset, PRAM Reset, and trying to reset the microswitch by pluggin in headphones over and over.... and changing the output in Audio MIDI settings. None worked.
What solved this for me, was going to Sound in System Preferences, then to Sound Effects, and where it says Selected Sound Device, it said Choose Sound Device. To my surprise, Internal Speakers was available. When I chose it, my Internal Speakers started working again, and came available as an Output in preferences.
Well, I tried everything except going to the apple store. (I had no red light and no Internal speakers option inside the sound settings)
The solution came from pelleindc and BulldoglMac. I believe that as you slowly plug and unplug your headphones while changing volume, the computer gets tricked and sends the volume beep before resetting to Digital out, but after disabling the headphones, right through the built-in speakers.