I have to be honest that I didn’t specially test this, but could it be that the videos simply don’t have sound?
With Lemma I simply parse the content of a post to figure out if it’s a image, text or video post. Based on the URL of the post I decide the show the video player, but that’s about it.
There is however a ‘bug’ that I always show the unmute/mute button, even if the video doesn’t have sound. The issue is that I only have a URL for videos, but no metadata if the video has sound or not. I’ll look into the fact if I can somehow recognize if the video has audio or not to prevent confusion, but that might take a while since there are more pressing bugs at the moment