The MacBook Pro's audio output port serves dual functions: as the analog audio out to external speakers or headphones, as well as a high-end TOSLINK fiber optic audio connector, which the Mac's Sound Preferences pane calls "Optical Digital Output." Sometimes after plugging in or taking out ordinary headphones, the connector starts thinking it's in digital mode and turns off the Mac's built-in speakers. There's a software workaround that fixes this problem.
As I wrote in a post several years ago, the issue is annoying. I have a new model of the MacBook Pro, however, the problem remains. Suddenly, the audio port is glowing red and the built-in speakers stop working.
The TOSLINK connector is just a skosh (0.5mm) longer than the stereo analog 3.5mm minijack. You can use a TOSLINK minijack cable or an inexpensive adapter that connects to less-expensive TOSLINK cable with plugs.
But the changeover to TOSLINK can happen at any time. It seems that when removing the headphones with a yank after editing some audio, I must have accidentally flicked a switch inside the port. The MacBook Pro thought that there was still a TOSLINK connector attached even though there wasn’t anything plugged into the port.
I found that a Japanese toothpick worked to flip the switch inside. Not the best solution.
However, the other day, I discovered a software process that also worked for me. It was in a post on Joseph Lorenzo Hall's Not Quite a Blog. I followed his procedure but I found a bit of ambiguity in it. So, here's a revised version that worked for me:
1. Close the Sound Preferences pane and Quit System Preference, if it's open. 2. Plug in headphones or a speaker jack. 3. Play some audio from an application. 4. Launch System Preferences and open the Sound pane. 5. Make sure the setting reads Headphones and then pull out the jack.
The name in the Sound pane should then change to Internal Speakers from the previously wrong Digital Out setting. However, I note that after performing this fix, my Sound pane says the Internal Speaker is of type Headphone port, which I'm not so sure is right. But at least the built-in speakers work.