How do I add backgroun music to a webpage via RV Sitebuilder?

You cannot do it in RVSitebuilder. You are able to create your own template from scratch and import to the system. It is not easy but not too hard. There are a lot of requirement for the templates. Once you create the template, you need to add the RVSiteBuilder element on it. Name it in the same way we do, and etc. Please check the document at http://www.rvsitebuilder.com/templateSpecification.pdf.