Try looking in the bottom right-hand corner of the Facebook page that you're trying to watch... There should be a little 🔊. If there is an x beside it then it's muted. All you have to do is tap on it to unmute
If that suggestion doesn't work, go into your settings, permission, and check to see if you have a mute/block/limited access on Facebook, or the URL the video is playing from.
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