Repent. Autoplay Facebook videos with sound are here.
It was just a matter of time
If you read that and let out an 'ughhhhhhhhhhh' you're in luck. You can disable it in settings. However, in typical Facebook fashion they're going to opt-in users to this functionality instead of letting people know they can enable it.
As soon as you hover over a video it will begin playing. On mobile sound fades in and out as you scroll through videos in News Feed.
You can opt out of autoplaying sound permanently by toggling off "Videos in News Feed Start With Sound" in Settings.