Nice Job.
As you have one speaker, you may want to include MP3player.setMonoMode(1); near the MP3player.setDiffertialOutput(1); as not to have there virtual sound effect.(what ever that is).
You may want to look at http://mpflaga.github.com/Sparkfun-MP3-Player-Shield-Arduino-Library/_file_player_8ino.html which you could easily adapt to auto detect and cycle through any file name, with the button.