@GeoffWozniak what is your desktop environment?

I have a desktop with an Nvidia GPU, with HDMI output (windowmaker, though using the normal gdm session manager, Ubuntu 18.04). I use pavucontrol to set the output to my headphones whenever it boots up (defaults to line-out with HDMI audio disabled). but, whenever screen blanking kicks in, something Nvidia-related (I suspect) re-enables HDMI output for audio, and uses that instead of the headphones.

I bought a laptop earlier this year that runs stock gnome (Ubuntu 20.04). my son uses it for his online classes once a week. no matter what devices he plugs in -- USB headphones, something in the headphones jack, etc. -- it never messes up like my desktop. he also has three possible microphone inputs (laptop, external webcam, USB headphones) and it always remembers the last selected device.