Not working for me as well.
Tried Recovery and it worked for a day and then stopped working again.
Also tried the "Tab audio muting UI control" flag. Nothing changed.
The only thing that I didn't do was disable extensions since I prefer them to not having mute tab working.
If I pin a tab then I can mute that tab and all tabs from the same site that I open after. If there's already a tab from that site opened before the pin that opened tab won't work.
Still works after unpin the tab if I keep at least 1 tab from that site open. When I close all tabs from that site then the mute stop working again and I have to pin a tab.
Pin tabs won't affect tabs of other sites.