There's an insightful post over at SEOmoz on how to convince a client their site does not need music (music playing in the background - not downloadable audio that a user initiates). It's good logic that can be applied to the "do it yourself" web designer/developer as well if you're considering implementing such multimedia. The reasons that stick out most in my mind are that it's obtrusive and uses up bandwidth. One issue not mentioned (or maybe not mentioned in enough detail) is that some users may have their speaker volume "cranked up" - the result of which is that obviously the disturbingly loud music may alienate your audience, drive them away and ensure that they never return. Since no one wants that you may want to avoid using background music on your site.