I have had the same problem for the past half hour and did everything suggested but wouldn't stop... until I realised that it was my Firestick remote which I hadn't moved from my table for days sending the mute signal. Pressed it once and all resolved! Just thought I would post in case yours doing the same is because of this weird FS issue...
The only thing that worked for us was to reset the Smart Hub and then factory-reset the TV. Apparently if the TV's internal storage gets close to full, it starts acting flaky. After doing this, we only reinstalled the apps we knew we'd be using. It's been more than a week now and everything is working flawlessly again: volume, mute button/icon, internet.
My issue was solved by repairing the remote to the TV. Assuming that you’re using the remote that came with the TV, you can do so by simultaneously pressing and holding “back” and “skip” until the repairing prompt appears on the screen.
