Apple - Display Mic volume in the menu bar
This app works for me: MuteMyMic. Available in the App Store at that link.
Appears to be free, as well.
It puts an icon in the menubar with a mic input level slider:
Disclaimer: no financial or other ties to the developer of this amazing app which I just found using Google.
Just wanted to share the following app too - https://apps.apple.com/us/app/mutekey/id1509590766?mt=12
- Good UI.
- Adds icon in the menu bar.
- Ability to add keyboard hotkey.
- Can select among input devices.
- Free.
Same disclaimer as earlier answer: No financial or any other ties to the developer. Just found the app on the app store and love it.