Is the video freeze on your screen? And before it was a very good add-on, but recently it causes YouTube to freeze wherever you pause or advance the video a bit. This is caused by the an other browser extension that you are using in your web browser. To fix this issue, go to the extensions page, and disable the other extensions you are using one by one except the Turn Off the Lights.
It hold the frame of that video content and you heard the sound is still playing in the background. That is one of the description that analyse there if a browser extension that manipulate the content of that video player. As a result, it is coming from another browser extension that is coming from the Chrome web store, Opera gallery, Firefox gallery or Windows Store.