One way to do it is...
<BGSound Src="song.ext" Loop="-1">
Obviously replacing 'song.ext' with your file and extension.
That particular loop value makes the song loop forever, I'm not sure what the other values do, I suppose '0' would simply be no loop... but just play with it to get what you want.
If instead you'd like the viewer to be able to play/stop or whatever the sound themselves, then...
<embed src="song.ext" autoplay="false" controller="true">
Would be your answer. Hope that helps.