I've been screwing around with this for the last couple of hours, and I'm fairly confident I've resolved the issue. But I don't think I can ever be certain.

The issue is related to StopTheMadness. I deleted the app and played several videos with no volume issues.

I re-installed the app and the issues returned. Same symptom this time, volume is reduced but not muted.

I should add here that I didn't take detailed forensic notes, because this is enough of a pain in my ass. But I want to say that the first or second video I played after reinstalling STM, each time I deleted and reinstalled it, played without issue. It was the subsequent videos, and every subsequent video that exhibited an issue.

Yesterday I had two videos play normally after reinstalling STM, which made me think the problem was resolved with the update. Today only the first video played normally, and the issue returned on the second and remained for each subsequent video.

There is a setting in STM to "Show native controls." Native refers to the standard video controls offered by Safari, with the volume control being a vertical slider at the right side of the frame. I had that setting enabled.

YouTube has its own control layout with the volume control being a horizontal slider on the left side of the frame, after the play/pause and chapter controls.

When I disabled "Show native controls," in STM I don't have the issue in YouTube.

To summarize, I recently began experiencing the audio being muted consistently at about three quarters of the way through a video, regardless of length. I would increase the volume and the video would play to the end without issue.

After that, I began getting notifications from YouTube that they'd detected an ad blocker, asking that I white-list YouTube or subscribe to Premium. I ignored those warnings and continued to watch videos and adjusting the volume after it had been muted.

This went on for several weeks. Last week, I got a much more specific warning that I would be allowed "three more videos," after which I wouldn't be allowed to watch anymore. In hindsight, I probably should have tested that to see if they would indeed somehow block playing videos. I think I watched at least four after getting the warning, but it kept popping up and was annoying.

I started my trial subscription to Premium (Google/Alphabet/YouTube will charge your credit card $1 to make sure it's valid, then refund that amount.) All seemed well at first.

Then I got a new problem, with the volume jumping to max. And it would do so repeatedly, after being adjusted.

Yesterday I updated STM and got another new problem, with the audio repeatedly being lowered, not muted, not max'ed.

So, three different manifestations of audio issues with YouTube.

For now, they have been resolved by disabling "Show native controls" in StopTheMadness. It appears that Google/Alphabet/YouTube want to control every aspect of your experience on their site. That doesn't appear to be the case on embedded videos.

Yet.