For those auto-playing videos in the news feed? They play without sound unless you tap on them

On iOS your application can not stay fully active for more than 10 minutes. After that time it gets into suspended state and can be waken up only when it receives a notification (and the wake up time also has time limitations). There are only three exceptions to this rule: active GPS usage for maps and trackers, active voice calling apps (for ringing) and music apps that are actually playing audio. Silent audio thus enables Facebook to stay alive forever.

This had also the effect that when you were listening to music or a podcast and opened the FB app it would stop playing your audio.