Does it still happen in a new profile? Alternatively you could refresh your existing profile. Make sure you stay signed out of your sync account when testing.

Thanks for the suggestion. I forgot to say in my initial post that refreshing FF ( as well as restarting it) does temporarily restore video and audio. I have set up a new profile, and am running it without any of the add-ins I normally use. (and keeping my first profile operational in another window). Sound and video are good after a couple of hours. I'll monitor and report back after a couple of days. Thanks again for your help

The problem does not arise using a new 'clean' profile.

My original profile has four extensions. I disabled them, and the audio/video problem continues. I have not tried removing the extensions one by one to see if the problem is specific to a particular one.

The extensions are : Adblock Plus Disable Javascript Easyfundraising Donation reminder Ecosia search engine

For now I'm happy to continue using a separate profile when I need to play audio/video. Unless there is a known problem with any of these extensions?