Microphone really REALLY soft after upgrade to 10.15.4
First, perform a SMC reset by following the instructions for your Mac. https://support.apple.com/en-us/HT201295
As commenters have noted, a Safe Boot is also a useful step. Hold down the Shift key immediately after hearing the startup chimes; you'll notice that login will take longer than normal, and you may see 'Safe Boot' in red letters at the upper right of the login window. Test the hardware meter again by opening System Preferences > Sound (or press Option and one of the function keys assigned to a Volume control).