Missing sound on/off button in Photos app

When I tap that icon that you show

it mutes the sound.

Have you tried a forced restart? 

Force restart iPhone - Apple Support

This is different from a simple Power Off. I had to continue holding the button down past the "slide to power off" message, and then I released the button only after  the Apple Logo appeared indicating a re-start. 

Let us know what happens…

That's not like the first picture you sent. What's going on?

I would suggest that you send a small screenshot of your phone that shows the entire screen. It would really help if you used sentences to discuss what's happening. Like, does this happen with all videos, or just some? Are these videos taken on your phone, or sent to you? Details are important. Going back and forth with one word answers to questions doesn't work--especially since you never answered my question about force restart for example.

I have a 16, and mine works as expected. What might be different about yours? For instance, do you have 3rd party apps that might be interfering? Have you used "Storage Cleaner" apps or "Speed up" apps or Video or Audio enhancers?

Does forced re-start fix it?