I'm running Debian Buster (recently upgraded) and using Pulse Audio Volume Control.

I have headphones connected to my computer (listed as analog internal audio), and I can select them as the output device for an application in the Reproduction tab. But every few seconds - which may perhaps be related to the cord moving slightly? - the selected audio device changes to my USB microphone - which has speakers connected to it - and then, one or two seconds later, it changes back again.

This behaviour began after the upgrade, it did not happen when I was running Jessie. It is a problem, as the audio starts coming out of one device or the other alternatingly and a sign with an icon for the device shows up in the middle of the screen whenever it changes.

Is there a way to stop these changes and make my system stick to the device I have chosen?

What you describe sounds like a hardware problem, a loose contact of sorts. Either inside the headphone cable or where the plug plugs into the machine.

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Maybe... But I had the same cable before the upgrade and my system didn't automatically switch to a different device every time it lost contact for a split-second. Is this behavior unavoidable in Buster?

I wish I had some suggestions, but I've never seen or heard of that behavior. I assume you've configured everything correctly in the pavu, in the configuration, output, and playback tabs, as well as the input tab.

I suppose so, although, as you may imagine, I did everything I thought was correct, so if there is anything I missed, I wouldn't know about it.

I think there is a loose contact which sometimes makes my computer stop recognizing my headphones for a split-second. Before the upgrade, all that happened was that I missed a split-second of audio, but now I get a sign in the center of the screen and the audio gets redirected to my speakers for a moment, and then back to my headphones.

Perhaps a more efficient driver. I really don't know, I've never seen that. Sorry to be no help.

That's OK, thanks for trying to help anyway.