Microphone really REALLY soft after upgrade to 10.15.4 Tested on Google Hangouts, Google Meet, Skype, Facebook Messenger, and Zoom. This is a MacBook Pro 14.3inch.

Even on this screen the microphone only registers when I talk VERY LOUDLY:


There were no issues before I upgraded. Since then, there has been another minor upgrade (which I applied). I've also tried with an external headphone that has a mic, and there was no improvement.

EDIT: Installed Windows on an SSD—with Bootcamp drivers—rebooted into it and the mic works. So it's definitely a software problem.
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).