Here is the fix for the Youtube & other site HD videos that play only beep sound / green screen:

After I updated my OS to Windows 10, and did these changes in about:config to get 1080p, some of the videos were making this sound, and some videos even had a completely green screen.
I did some digging and I found out that it was "media.fragmented-mp4.exposed" that was causing this problem.
So all I had to do was to turn everything to "true", and leaving "media.fragmented-mp4.exposed" on "false".

I was then able to view 1080p videos without any problems.
So the fix was in my case to open new Tab in firefox and url is: about:config
and there search for above mentioned variable and make it "false". Refresh page with video.

Source: https://support.mozilla.org/en-US/qu...#answer-762563

This is also related: http://superuser.com/questions/95800...and-beep-sound