When I click on a YouTube video, a duplicate audio begins playing a few seconds before the actual video starts. When the video plays, the video audio is in sync with the video but the duplicated audio plays in the background out of sync. I tried looking at add-ons and tried uninstalling and reinstalling Firefox. I tried other recommended fixes but the problem persisted. I did not have the same problem when viewing videos in Internet Explorer.

I discovered a simple solution. I had sometime back installed another video program called VSL Video. I never used it and had forgotten it was installed. Apparently, VSL Video was trying to run in the background every time I would use Firefox to view YouTube videos. I did not have the problem with Internet Explorer. I uninstalled VSL Video and the duplicate audio problem went away. I hope this helps someone with the same problem.