I read a "horror story" about a gun store that hired some friend of the owner to build their site. Imagine about three seconds after the homepage loads (and you have iTunes playing as you work) suddenly the office is filled with the sound of automatic gunfire (too likely an occurrence, to be taken lightly these days). Poor fellow was looking for ammuntion prices for an upcoming hunting trip (all perfectly legal) and scared three co-workers so badly they nearly quit their jobs.
Granted, your client probably doesn't want anything like that, but one of the quickest ways to drive people away from a site is the have an annoying click, or swoosh every time they navigate to a different page.
Basically what they want is just a little ding when you either rollover or click the button. I also am very much opposed to automatic sound playing without the user initiating it. However, I must admit I have been bullied by clients in the past into doing it.