I figured it out. I was using Processing provided by the Mint Software Manager which turns out is a Flatpak. I used that as it was identified as the same version of Processing (3.5.4) available as a tgz from the Processing website. At least for me, the Flatpak bundled libs dont work with the sound library.
I removed the Flatpak, downloaded and extracted the file from the Processing website and it all works properly!.
(As a note I generally try to install from the software manager when possible to ensure dependencies all get taken care of.)
-Rob A>