Hi mtthw,
Welcome to the community 
For what I can conclude from your description, it must be a codec issue.
Both clementine and rhythmbox are depending on gstreamer
VLC and audacious both depend on libav.
It is, ofcourse, a bug. But since 18.04 is end-of-life there is not really a chance of getting it fixed.
However: You could try to install the "gstreamer1.0-libav" and see if that fixes things.
This is a gstreamer-codec that is actually a wrapper around the libav codecs.
It just might do the job.
sudo apt install gstreamer1.0-libav
Disclaimer:
Now, I used to run 18.04/32bit on my almost 18 years old single-core+HT P4/32bit laptop but I changed it to Antix when 18.04 went end of life, 20.04 did not support 32-bit at all and Trisquel/32bit (MATE desktop) turned out to be a bit sluggish due to gtk3. So I am not able to test anything I propose here. Also, I've never used Clementine or Rhythmbox.