Useful, but I suggest a few improvements, which you may try in "Sleep Timer Fix Reload" by Miguel (sleep-timer-fix-reload). Do not hesitate to integrate the changes if you like them.

The main new feature is a new option, set as default, to "Reload" the tab rather than "Close" or "Mute" it. I believe that this is often more useful, so that the tab stops playing music or video but is not lost (useful if, later, you want to listen to another song or program on the same web page). Initially, I wanted to implement "Unload tab" instead, but I couldn't find the way to do it in the API, and really the result would have been quite similar to "Unload".

I also made a few minor improvements, such as preventing pop-up from being reinitialized when finished, and having the pop-up displayed at a more reasonable size on smartphone (using META viewport in the HTML).