Well, how about this for a coincidence.. I've been futzing around with this for the last two days, and not making any progress. Tonight I received several updates on Linux Mint, which included the kernel and 'alsa-ucm-conf'. I thought I would give it another try and - behold! WSJT-X is now sending audio!

FYI, I have the 'alsa_input...' and 'alsa_output..." audio devices selected in the configuration menu.

Cheers,
Steve K4RVA