I know it's not really a proper solution, and in an ideal world advertisers wouldn't stoop to such tactics, but why not just turn your speakers off?
I don't even bother to switch my speakers on unless I specifically want to listen to some media clip or other - the rest of the time, I'm listening to music (from a proper stereo, obviously) and don't want annoying beeps and twangs every time a dialog box pops up. Try it for a while - you'll probably find you don't miss the sound at all.
"Multimedia" - pah! Who needs it?
cbl.