You're totally right about the mic + speaker icons. I wonder why they did that, if clicking either of them apparently opens this shared menu. A speaker as a representation of "Sound" with both mic/speaker settings inside doesn't strike me as too abstract. Or if they really want separate symbols, maybe they could be visually joined somehow.

And none of the other screenshots show the microphone icon, it's only in that one. Only shows up with a mic connected I assume.

Or could the mic be there to give a "something is currently using your mic" indicator?