Step one: Stop tearing.
We published a suite of tools to get you past ACX Automated Quality Control (the Robot).
https://forum.audacityteam.org/t/audiobook-mastering-version-4/45908/1
It requires some extra downloaded tools and installs, so you need to be comfortable with that. The tools depend on each other, so don’t leave any out.
Generally, it’s not the microphone. It’s always the room. Can you tell if the computer is on just by listening? Can you tell if the refrigerator is on? Can you tell when the MetroBus goes by (my personal favorite)? Those are very serious problems because Noise Reduction is not the global rescue it appears it should be, and some noises don’t reduce.
Record a test clip and post it here on the forum.
http://www.kozco.com/tech/audacity/TestClip/Record_A_Clip.html
If you have real/serious problems, we can tell you right at the top and avoid cyclical troubleshooting.
Koz