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It seems Skype for Business fails to register itself with MacOS's privacy settings. There is a comment here that describes a workaround. Unfortunately, manually editing the file in question seems no longer possible on more recent versions of MacOS. The result is that the mic can still be selected in Skype for Business, but the OS doesn't direct sound from the mic to the app.
Usually, Skype For Business should register itself here (and be active):
System Preference > Security > Privacy tab > Microphone menu
NB: It does still register for camera for me.
Anyway, it's a bug that needs fixing.