One thing to keep in mind about older versions of the nvidia proprietary drivers is that they will only work with specific kernel versions (and specific X versions—not sure about Wayland). Once the driver series your card needs stops being updated, you can’t update your kernel without patching the driver. Assuming you have the skills to patch the driver, or someone who does makes their patches public.

I went through this song-and-dance with a very old laptop that had a card of the NV40 generation as its only GPU (no integrated graphics). Eventually I did install nouveau on it, and used it for several years without any issues.

TDE has this natively under the advanced window settings, so I would expect KDE to have it too.

Or ditch udisks in favour of pmount (or udevil?), which shouldn’t be affected as far as I can tell. That will get you a few months’ grace before a similar problem pops up there.

Yeah, that’s how I was taught in school in Canada in the 1980s, although no one ever explained why. It always did seem odd.

What’s the output of cat /proc/asound/cards on your system?

The 6.6.x kernel series is LTS and should be fine as a downgrade target (6.7.x not so much so). Unless there’s something specific from the newer kernel versions that you need to drive that system, there shouldn’t be any issues. I’m still on a 6.6-series kernel.

That being said, you could try troubleshooting this from the bottom up rather than the top down.

First, use lspci -v to verify that the device is being correctly identified and associated with a driver.

Next, invoke alsamixer and make sure everything is unmuted and your HD audio controller is the first sound device. The last time I had something like this happen to me, the issue turned out to be that the main soundcard slot was being hijacked by an HDMI audio output that I didn’t want and wasn’t using, and that was somehow muting the sound at the audio jack even when I tried to switch to it. A little mucking around in ALSA-level config files fixed everything.