@Dan Murphy Is this on your phone or on your computer?
If it's just on this one browser, it's probably specific to either a browser setting or your
YouTube account setting on this one browser. If it's happens on ALL the browsers you have, then it's possibly a Windows setting issue.
Have you tried signing out of your Google accounts and your YouTube account? Deleting those cookies. Reboot. Do NOT sign back in. See if it happens. If it doesn't happen then it's in your Google/YouTube
account settings.
Also, temporarily turn OFF the AdBlock Plus while you are on this thread, to see if that fixes the problem. There should be a button in the top bar of the browser with a number of how many ads it's blocking.
See if that fixes anything.
As a quick fix, turn off the volume button of a YouTube video. It's in the bottom bar of the VIDEO. Close the page. Reload the page. Even if they start playing again. All the videos should be muted. To listen to one video, turn the volume on for that video only. Turn it off when done playing, so the others stay off when you load a new page with a YouTube video again.