Audio through *wired* headphones crackles, stutters, pops, cuts out

I have one of the new M1 Mac Minis (16GB RAM, 256GB SSD). I've seen various flavours of this problem, usually having something to do with Bluetooth not working, but the usual fix is to try the wired headphone out. I don't use Bluetooth at all and have turned it off through System Preferences.

My wired headphone out is the problem. I can't play music through Spotify's app, anything like YouTube through Chrome or Safari, without experiencing problems with the audio. (It stutters, or crackles, or pops, or cuts out very briefly.)

It seems to be worse when the machine is under some load.

This didn't happen when I first got it, so it might be software, or "age"? Has anyone else experienced this?