Chrome: Chrome recently added a handy volume indicator that shows you which inactive tab is making noise, but you still have to open that tab to actually silence it. Mute Inactive Tabs is an extension that does it automatically.
With the extension installed, any tab that’s in the background won’t be allowed to make noise. For videos, the extension usually pauses the video, while most Flash sites just aren’t allowed to load at all. Obviously, this doesn’t work great if you run music or podcasts or whatever in the background, but if you’re sick of background tabs suddenly blaring noise, this stops it from happening.