Perhaps Google is getting more money from those advertisers who can post ads with sound. So I doubt that they would give control to viewers to be able to disable it. Honestly, I just keep my speakers off unless I have reason to specifically turn them on, so I would never notice something like that.
But to directly answer your question, no, there isn't any way I am aware of to block audio from Google ads.
BTW, what ad types can even support an audio track? SWF?
I do hope that Google isn't going to shoehorn multimedia ads (video and audio) onto ad hosting sites, otherwise the burden may become intrusive enough that I would have to consider an alternative to that income stream. Not that this site brings in much revenue that way, but heck, every little bit helps.