Just go into Mail preferences and under the pull-down menu for "New mail sound", just pick "Add Sound". On the pane that opens up, click "Add..." and navigate to the mp3 you want to play.
I would hope you either have very short mp3s to play or very few e-mails though...
not sure the number of items you get a day, but this for me would not work and become ridiculous. i get around 20-50 personal emails per day, so i couldn't imagine listening to the same song that many times.