mute microphone on new macbook pro

I've got a new 2018 MacBookPro, and am initiating Screen Sharing with my wife to help her in another room in the house. The mic is on by default on my end and her's. On her end, she can mute it, but I can't figure out how to mute it on my end (on the screen sharing initiating end). There also seems to be no way to just mute the microphone locally. Some suggestions in other questions along these lines, is to choose the line in audio input, but that apparently doesn't exist on the new MacBook Pro's.

How do I mute the microphone in just a system-wide way? Or how do I mute it just in screen sharing?