There is no video playback sound on some Facebook links. I tried it on my friends iphone 5S and had the same result. Can anyone who has liked the Fox News newspage on Facebook attempt to watch a video?? There is NO sound! Wtf?
Sounds more like an iOS issue rather than an issue with the iPhone hardware which is what this forum is for. Given that you posed in the wrong forum might be why you don't see anyone complaining about it.
The problem I always have with Facebook is when I start playing a video and then close it it's still playing in the background and I have to reboot the app... Facebook needs to fix their crap