There used to be an extension to stop youtube videos from running/loading, but it is gone. Also, I noticed Android's Chrome, has autoplay blocking, built in but Desktop doesn't. Along with flash blocking, via Extension (if exists) or internally which exists, by default. You can install Content Blocker Extension and perhaps to whip your problem.
What I remember is that in the OLD OPERA versions back some years, had a plugin blocker option built-in, which seemed to be cool as to ask you if you wanted to play the content, and it stopped all the videos on pages that I visited. Sadly to say, this Option seemed to disappear with the Upgrade to HMTL 5 based browsers. I used to prefer OPERA just for that reason.
I actually used Script Blocker Extensions to stop some videos from playing, like the annoying CNET downloads.
you can try this 4 star Video Blocker Extension, says it blocks videos in general from playing.
my alternative to not hearing the videos, when not blocking Extensions is to Enable SOUND blocking Option in Settings.