I have a mac, and I couldn't find a defragmenting utility that works on FAT devices.
I also tried moving all the songs in and out of the SD twice (three times if you include when I backed up my files before formatting)
Interestingly enough, redownloading the songs worked, which didn't work the last time this happened.

In brawl, some of the custom brstm music I use turns to static at a certain point. It always happens at the same part of the song. They used to work, but they suddenly stopped working. I've tried formatting the SD card, but it just makes the static sound different. I'm pretty sure that it isn't a bad brstm, because I got them from different sources.