It's been doing this for at least a month or so, but I haven't made any changes on my end during that time like new extensions or anything.
Video will be playing normally, then randomly the start of that video's audio will restart as a new track. The original continues progressing with the video, but whatever audio they used for the intro just butts in and now I'm hearing two iterations of the same video almost like it's running in two tabs.
Refreshing the page fixes it, but navigating from that page to somewhere else in YouTube does not - like I can return to the homescreen and the original audio track cuts out as expected; but the one that started randomly will continue to play, even on the home screen, until I refresh or navigate to a different website.
I assumed this was more anti-user bs aimed at firefox users, so I've just been tolerating it until a fix comes out; but searching the web I don't see any complaints about this, so I think it's screwy on my end.
Windows 10, Firefox.
Edit-
Oh, and sometimes it will start randomly playing audio in the homescreen of one of the videos that appears there. I don't actually click any of them, I'll just be scrolling through them, and suddenly "HELLO INTERNET! WHACKADOO BOB HERE WITH ANOTHER--" refresh page to shut him up. This started around the same time.
Idk if it's trying to do a mouseover preview or something, but it'll be a video I scrolled passed a good minute ago and just randomly starts blasting.
/shrug
Shot in the dark here. Do you have any extensions running? Try disabling them, and see if the problem persists. I often run into YouTube weirdness that's the fault of an outdated or broken extension. Especially ones that affect sound, video, ads, or element blocking.