It wouldn't be a sweat with php but I'm guessing you want it plain HTML huh?
A little googling turned this up:
http://www.smartwebby.com/web_site_d...play_sound.asp
Hope that helps.
Although I would like to note that almost everyone hates background sounds on the web and they only sound cool by the owner of the website. I think it's a good thing that you are giving the user the control to decided wether or not to play it though.
Oh an no MIDIs too... that's good.... MIDIs are evil....