Some people have too much time on their hands. Sometimes this leads to some interesting creations, like a custom Super Mario World stage that plays by itself.
If that's not good enough for you, how about an autoplaying level that syncs to some music?
If you'd like to see more of those...
Or if you prefer focusing more on the music itself, let's enjoy some Mario Paint.
http://www.youtube.com/watch?v=kIrsJFEb7pE&NR=1 (My favorite)
This one has a small bunch of other videogame tunes played the same way.