The other night I let our three year old watch some videos on my MacBook Pro. Since we were sitting in the living room in the midst of things, I had him wear headphones. All was well until the next day when I realized that my computer wasn’t making any sound. When I looked at the headphone jack, I noticed that there was a red LED light glowing inside. After digging around on the Apple Discussion forums, I was able to solve the problem.
In short, it seems that there’s a little switch inside the headphone jack that toggles between analog and digital sound, and it was stuck such that my computer was trying to provide optical audio output. I tried plugging and unplugging the headphones, but the red LED remained on, and my computer remained silent. I also tried to play around with the sound settings as well as the settings in the Audio MIDI utility, but the options that I needed were greyed out.
The solution that I ran across was to gently insert a toothpick into the headphone jack and wiggle it around. More specifically, work the tip of the toothpick around on the bottom of the headphone jack. In my case, it took just a second or two to jiggle the switch back to its default position, extinguishing the red LED and restoring sound to my built-in speakers.