Curious. I don't happen to have my SX-2000 THB handy, but in the SX-200, from LW 16 all the way up to ICP, the Superkey is where you would find the Mute key. Or in Form 9*.

You are correct that "Microphone" is NOT the correct key, as that is for silencing a speakerphone only. My advice would be to review the Technician Handbook.