This option already exists. If you go to a website that has autoplay blocked, you can toggle the permission from the lock menu at the left side of the location bar (see attached screenshot).

I'm running 66.0.3 and I don't see that toggle. I set the "Block websites from automatically playing sound" setting and when I go to a site that has a video with sound (that is not autoplaying), there is no sound when I play the video; I have to go in and add that site as an exception. This happens on both Windows and Linux as near as I can tell.

You can click the block auto-play icon that shows in the screenshot Wesley posted to the left of the Control Center 'i' icon to disable the block. You can re-enable (Allow/Block) auto-play via another click on this icon.

That's what I'm saying; I don't have that option in that area. Video plays but no sound. Video plays with sound in chrome.

Maybe it's something on my end, though I can't imagine what that'd be. I would like to thank everybody for your help.  :)

I think it was some problem in my profile. I started with a clean profile and I got sound (still don't see the block autoplay icon yet). signed into firefox sync, let all settings get downloaded, restarted and tried again -- still getting sound. So I'm not sure what happened to the profile.

Thanks everyone!