What OS.build.platform are you dealing with?
Are any mp3 files in use?
are they in the directory you are running the command in?
Try this experiment in an empty directory
(I use mp4 here in case valid file[n].mp3 exists in /var/www/backup/files/ already)
for i in `seq 1 5` ;do touch file$i.mp4 ; done
mv *.mp4 /var/www/backup/files/
Please let us know...