A regular problem for me with my Macbook Pro is getting the sound disappearing because the headphone jack gets stuck in optical mode (TOSLINK on). You can tell it’s happened because:

1. a red light is on inside the headphone jack (the light is the TOSLINK signal); and
2. adjusting the volume through the sound controls brings up a sound meter with a “no signal” symbol below it (looks like a do not enter sign).

What happens

The headphone jack serves two purposes: a normal audio out and TOSLINK out. Inside the jack is a very small switch that turns on and off the optical audio output while simultaneously disabling the internal speakers. The switch can be turned on by simply using headphones. Almost any 3.5mm TRS audio plug can cause the problem, although some are worse actors than others. The design of the component that Apple uses is known to be poor but Apple doesn’t have to fix technical problems because it’s a design company and bad engineering never hurts them.

The Solution

Setting aside the fact that making it do double duty is stupid design, here’s how to fix it.

Get a toothpick. You can use an unfolded paperclip but wood is less likely to damage the innards.
Look inside the 3.5mm TRS/TOSLINK jack for the little switch. The TOSLINK signal acts like a nice flashlight.
Find the switch in the back at about 12 o’clock, looking straight in with the laptop sat flat on a surface. At least it was at the top of the TRS tube in mine.
Gently probe the switch with the toothpick and try to get it to turn off. You will know that you’ve been successful when the TOSLINK red light turns off.

That’s it. It drives me nuts on a regular basis and each time I have to paw through endless unhelpful complaints in Apple forums to find the answer. I figured I would put it up here and make it easier to find for everyone.