It may be that the lack of sound on Facebook videos is by design and is actually the device respecting your wishes. How? Because it's common to accidentally switch on the mute switch on the side of your iPad or iPhone then, even though you tap a video, it plays in Facebook muted.
So that’s the solution, just flick the switch on the side of your iDevice and get sound back on Facebook videos.
Note: Not all modern apple devices have a mute switch. What might be the problem is a glitch in the app. The solution would then be to:
- double tap the home button and slide the facebook app of the screen to force-quit it, restart and see if Facebook videos are still muted when you play them.
- hold your finger on the facebook app icon until it wiggles, then tap the (x) to remove it (and then reinstall it)
If your device doesn't have a mute switch and facebook videos are muted while playing, just make sure you aren't worried for no reason. This article was meant to help those who, after they tap on the video it don't hear any sound. These days Facebook has the videos muted on the main page (called the feed) so videos won't play with sound automatically. If you want to hear what's going on you have to click/tap the video.
In the case where you have accidentally turned on the mute, here is the mute switch for the iPhone: (may be a bit different depending on model)