My Zumo 660 can't find mp3 finds on my micro SD card, yet they are visible when I plug into my computer and I can play them from the SD card in my Zumo via the computer (either iTunes or Windows Media Player).
Zumo can find the mp3 songs when I move them to the Zumo's internal memory, but that is not a correct solution.
Jim Puckett
Garmin GPSmap 276C, 478 and 640 owner plus new Zumo 660