Have the HD Zumo 660 & have the same problem - sometimes. At times it finds the MP3 files, other times it does not. Have a couple of SD cards & makes no difference which one. One only has about 300 songs. Take out the battery, the card, and after you replace it, the unit will find the MP3s.
Reported this to Garmin - long story but had several units (sent by garmin as replacements) until I got one that worked & half of them did this. So Garmin tech is aware of the problem, but my experience with them is - they suck & they do not care. They also lie & during my swap out units caught them in several. Customer service at Garmin tries to be helpful, but their tech people are knuckle dragging snail brains. Other than that they are the offspring of goats bred with dung beetles. They are efficient at telling people to: update your software & turn the unit on & off. Duh.
If you search Zumo Forums you will find this as a listed problem. One suggestion is to wait for 2-3 minutes for the unit to "find" MP3 files. This works - sometimes. All my songs are in folders of about 25 songs each. Worked GREAT on my old Zumo 550. Same SD cards. At times I miss the 550, but when the 660 works, it is a better unit.