Strange behavior with Scarlet Solo USB i/o

I have a studio system consisting of an old, amazingly reliable HP
8740W laptop with a Scarlett Solo USB mixer attached.  It's running
Xubuntu 20.04.3 LTS with Audacity version 2.3.3 (which, strangely
enough, I cannot get through an SSH connection because audacity
insists on connecting to the display even when run from the command
line for version info only - separate issue).

The laptop has separate headphone and mic jacks, built-in mic and
speajers, and works fine with a logitech USB headset (mic and
headphones).  It also has an HDMI monitor connected to the Display
Port via an adapter, and Audacity gives me a dizzying array of
multiple mic and speaker options - way more mics than speakers.

For quite a while now I had been unable to get either the mixer's mic
or the mixer's headset to work from within Audacity.

Tonight the mic started working beautifully, but the headphones get no
sound at all.  When I set the audacity speakers for the one mixer
"speakers" option, there is a sound like a jack being plugged in (kind
of like a 10Hz rumble), but when I click the play button, it makes one
"click" at the start of the playback and then no further sound comes
through the headphones, or anywhere I can hear.  If I set it to pulse
or sysdefault, I get sound through the onboard speakers just fine.

This is for a voiceover recording studio, so the mixer's mic and
headphones both need to work properly.  There's a huge difference in
the sound between the onboard (tinny) speakers and a professional
headphone set.

If I use Zoom as a test, both the mixer's mic and headphones work just fine.

What am I missing? Any help or suggestions would be greatly appreciated.

Thanks in advance.

Mark Richter, Lead Software Engineer
Award-winning Author, "Mystic Prince"
Avid Xubuntu user/admin/fan
http://www.linkedin.com/in/markrichter1
Registered Linux User #472807 http://counter.li.org/
FSF Member #12694 http://www.fsf.org