Is it possible for a Safari extension to prevent hidden tabs from playing audio?

I was watching a movie last night in a Safari window, and suddenly 2 YouTube videos started playing in the background. It was a gigantic pain to find them and turn them off (since I typically have a lot of tabs open). I know it's possible to disable audio from any given tab _once I've found that tab_. But that's equally as much of a pain. Since I don't have the power to punish the guilty parties at YouTube who have no business making their site do things like that in the first place, and manually disabling tabs doesn't really solve the problem, the only acceptable solution seems to be to find a way to automatically silence all tabs that are in the background. This sounds like a job for a Safari extension. Is it possible for a Safari extension to automatically turn off sound for any background tab? (And turn it on again when the tab is moved to the foreground.)