The Wrong Way
There are SEVERAL guides online telling you how to normalize audio to provide a great listening experience by using FFmpeg's "loudnorm" filter. Most walk you through a simple process called "track normalization." You set a goal loudness, and adjust every piece of audio up or down to match that loudness. This is evil and immoral and I'll tell you why:
Loudnorm changes the way things sound!
Loudnorm crushes dynamics!