My question regards available hardware... an adapter
taking a USB plug_ at end of a mic cable_ which adapter
would then connect to the computer's standard
audio/mic input.

Anybody seen such an adapter ? After looking an hour
or so this morning on USB gadget sites, I found lots
of the audio-mic-plug-TO-usb adapters, but this again
is the opposite of what I need.

The problem Im having is with older versions of
a sound app I use (audacity)_ newer versions of
this specifically support USB mics_ but_ that
feature is missing in older versions.
Im still guessing really, but assuming audacity will
treat any legal connection via audio port as "mic"

Just to be clear, you have a USB microphone that you want to convert to a standard line-in? There is almost certainly no adapter to do that, nothing that I have ever seen anyway.

But I am not sure why the app would need to specifically support USB audio devices either. I would imagine any audio input recognized in ALSA should work.

I myself had considered whether a usb-port-powered device
could do well thru a standard audio port... and... you seem to
be emphatically opposed to this possibility(!). After a day
of searching, I'll have to agree... there probably isnt such
an adaptor made.

As for "why" an app would need to specifically support USB...
hmmm... dont know enough to answer, except to say, my older
Audacity simply will not work with this USB mic, whereas, in the
more recent version, the same USB mic shows up on a menu, can
be selected, and does, of course, work very well. Possibly,
USB microphones were not available when earlier versions of
Audacity were developed(?). Thanks for replying.