I placed a few mp3 files into the microSD main directory. I didn't format it. There was also a Garmin directory already on the memory card main directory. I didn't use the Garmin folder. I presume that the insertion of the blank card into the GPS caused an automatic Garmin folder creation on the card, or it was done while attached to a laptop.
When the Zumo was disconnected from the laptop, the files show up under the media player on the Zumo screen, divided by artist and album. When I connect the Zumo harness to the bike via bluetooth through the aux input, I'll see if the mp3 files play through wired J&M headsets or the external speakers.
Thanks for the help so far, and the link to the Zumo user forum.