I'm sorry to hear of your frustration surrounding this matter. By default, videos will play in a muted state in the Gallery app, and will require you to either tap the speaker symbol in the bottom right corner of the video, or tap the Volume Up button on the side of the device to play the video's sound.
Andrew L Why can't Samsung give us the option to choose how we want it to play ?
How can I send a video with volume not muted on whatsapp?
I've turned off auto-play in Samsung internet settings (not that I use Samsung internet) but can't find un-mute in gallery anywhere.
This is annoying to say the least.
Samsung please fix this