Veritas59's adblock add-on is really nice if you're using Chrome and works for this exact issue you're wanting to address.
I will also recommend the Chrome add-on of "Disable HTML5 Autoplay." This will keep embedded videos from automatically playing when you visit websites. I hate videos that automatically play on my news sites and stuff. I sometimes play music loud and forget to turn the speakers down, go to a website, and BLAM! Some loud video scares the hell out of me.