Laptop inconsistently detecting and using external audio (heaphone jack) — workaround

Searching to solve this problem i found lots and lots of other problems that didn't quite sound like mine: My headphones/speakers jack seemed to have developed a loose connection for Lenovo T420 running Debian Mint. Sometimes it will stop (or not start) playing audio through the headphones, and fall back to playing through the internal speakers. It's not registering it as headphones / external audio that should override the internal speakers option, but, crucially, system beeps and testing the external audio output always worked even as something else continued to play through the internal speakers. So it's not that the external jack is ever not working, it's just sometimes ... not telling the system it's working and should be used? Anyhow, my symptoms:

• Problem persists with different headphones and speakers.
• In my case the first few minutes with a new application (after a suspend?) or after a reboot would be good, and it would get increasingly flaky, until it would stop coming out of the speakers/headphones at all.
• Also in my case, the problem came out of nowhere — was listening to music, left the computer and unplugged the headphones, came back, and no more sound through the headphones, just the internal speakers.
• Unplugging and plugging in the headphones has no effect whatsoever, although fiddling with the power cable sometimes seemed to toggle which output it used (though that's likely a case of random feedback leading to illogical conclusions.
• This behavior persisted through reboots.
• But if I tested the sound for the external output, it always worked through the headphones.

The dead simple, manual override, workaround that worked for me on "Linux" Mint Debian Edition with Cinnamon:

1. Open Sound settings (you can get there from the volume control widget or System Settings)
2. Under the default Output tab, under Device, click "Analog Output Built-in Audio"

That's it. You're done. I keep the settings open for easy switching back... and so it doesn't forget and start trying to figure out what device to use on its own, and screwing up, again.

You'll need to manually set the device (internal speakers or external) that you want to use each time, but for me two seconds doing that is better than another several days of searching and at best, it seems, setting output to one or another and changing only after an update. I've not seen any fix for the root problem of flaky detection.

Note: If your problem was the external speakers not working at all, you need another fix, perhaps this or elsewhere on thread i also offered this workaround to.