YouTube shouldn't autoplay videos. I especially hate it when you click on a channel profile to check out what they're about and some promo video starts up at max volume (my computer happens to be sensitive to sound differences, too). At any rate, YouTube should only play a video when the user presses the play button.
C'mon on, you must know that the user's expectations of YouTube are different than most sites.
Even on youtube, autoplay is annoying.
Especially when you re-open some browser session and then one of those tabs happens to be a youtube one playing some video.
That shouldn't happen in Safari or Chrome anymore - videos don't autoplay until you actually open the tab for the first time.
I have YouTube set to not autoplay.
The plugin "HTML5 Video Everywhere!" would be worth it if just for the ability to disable autoplay.