As Tapeworm said, those are Finder files that the Mac system uses. They all start with ._ (period underscore) and are 0kb in size. Your Mac needs them, so don’t take them off your computer.
Because in its wisdom Apple decided to give the finder files the same extension (mp3) as the actual files, the Sansa counts up the number of mp3 files and thinks they are songs.
One way to deal with them is, after you’ve transferred everything, search the Sansa (or microSD card) for ._ and delete them all. Or sort the files by size and all the little 0kb files should come up together, and you could delete them from the Sansa.
There are probably automated programs that do the same thing if you want to Google around.
