Chromecasting from your laptop or desktop is great, install a Chrome Extension and you’re good to go. What isn’t great is auto-playing embedded YouTube videos, which what has been happening for Chrome users who have the Chrome Extension installed.
According to a Google Engineer, who commented on the Chromium Bug Tracker the issue is apparently due to a ‘bad push’ yesterday. It’s being looked at, with a fix being qualified and scheduled for release ‘today’. In the meantime, you can uninstall the extension, restart Chrome (some people have had to reset their Chrome settings), and you should be good to go.
It’s annoying, but it happens, relax and just remember in the meantime to search for the small speaker icon at the top of tabs when you want to see which one has a video playing on it. The fix is due today, so hopefully you’ll be Tab-casting again in no time.