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SOVED:
Having the same issue and while restarting the Plantronics Hub software during a conversation does supply a work around, I had a Co-workers headset that was set still working to compare to.
Comparing Plantronics HUB versions (Help -> About HUB), we noticed that he had an older version 3.22 (his was asking him to update) where mine was the current 3.23. Went to Plantronics website and found the older version of HUB (3.22) and installed that on my machine (shows 3.20 once installed - thats a bit confusing). Mute is now working again. I suggest do not do the HUB update to keep it working.
I trust mine got updated and that's when it quit working.