mplayer lacks an option for selecting files in a reverse order. So here is a quick way to playback all mp3 files in reverse order:
$ ls -1 -r *.mp3 > mp3.rev
$ mplayer -playlist mp3.rev
- -r reverse order while sorting
- -1 list one file per line
- -playlist file : Play files according to a playlist file
Let's hear your shell hack in the comments.