A few months back Facebook started auto playing videos with sound. Turning the sound off on one video used to stick and would work through all videos and multiple launches of the Facebook app. Not so much after the last couple of updates.
Facebook now (as of a week or so ago,) turns audio on at each launch instance (you can turn it off, but once FB is gone from memory and relaunched it will be back).
Click the hamburger stripes (top left,) scroll down to app settings (near the bottom,) and turn off “Videos in News Feed Start With Sound”.
Once again you can browse in a quiet place without the scream-jump picture videos getting you and everyone holding scalding hot coffee around you.