This is a tough one to research - everything I turn up is advice for the end user. There is a complaint thread on Google's own forums about the issue with Adsense, but so far the only advice there is to go to text ads only... not exactly a solution.
I learned that there is an actionscript method called stopallsounds() - but that would need to be embedded in the Flash file itself. Since the files are supplied by a third party, that's a no go - and not all sound ads are going to be Flash at any rate.
Right now, I assume that the responsibility falls on the third party - the ad network itself. It seems like there ought to be something you can do on your server, too, but I can't find it. Hope someone else can turn it up for you.