In my experience, sounds stop on page turn. Can you place the sound on each page?
Actually yes, But If your export to Interactive PDF option, InDesign allow you to don't stop the sound with you turn the page, ePub desn't.
I read it should change in html code. Weird that we have a big problem just to add a sound as background while you're reading a ePub.
What a shame Indesign doesn´t work properly yet in ePub.