I haven't ever seen a single line of Bash before, but since you PM'd me, I'll give it a shot.
If you are trying to rename all of the mp3s to have the prefix "band_song", then I would have to guess this. This is a total shot in the dark though...
#!/bin/bash
for x in *.mp3
do
mv "$x" "band_song$x";
done
Unless your script already worked, and you were trying to "check the input". Which you'll need to be more specific about.