Facebook has just implemented a new feature that allows any video you encounter to automatically play as you scroll down your Facebook newsfeed. If you choose to click on it, it will also play the sound. If you are bothered by the automatic video play in Facebook, just follow these simple steps to disable it.
Settings -> See more settings -> Videos -> Off
Or, if you are too lazy you can always use this link: https://www.facebook.com/settings?tab=videos