UPDATE (Feb 11/2014): Looks like Chrome now has this feature. Not sure about other browsers though. Original post as follows:
It happens to everyone. You’re surfing the web and you have several pages open when SUDDENLY a loud autoplay ad starts up; it’s invasive and you’re scrambling to find it and shut it down. Another scenario is when you accidentally hover over an ad and you are bombarded with sound, and you cannot mute or lower the volume. Drives me crazy.