Thanks for your response.
The standard options line in /etc/modprobe.d/alsa-base.conf for this card seems to be:
options snd-hda-intel power_save=10 power_save_controller=N
which seems very environmentally conscientious but doesn't manage the card properly.
I would think this sound card jacks issue would be a deal breaker for any users like me who are interested in music creation. I was certainly very disappointed to discover I was going to have to spend hours and hours fiddling with system config issues just to record audio. I was used to those horrors with converted windows machines, but I thought buying a System76 box would mean smooth sailing. I'm a bit dismayed to learn that System76 apparently doesn't consider it very important that all hardware work out of the box.
It's nothing personal, Tom, I appreciate your attempts to help. But this episode has made it less likely that I'll but a System76 machine in the future.