I've got some AVI files that I've processed via VDub, but the audio is too low on all of them... to different degrees. How do I normalize the audio without having to re-re-compress the video?

Years after the main YouTube app got the feature, YouTube Music is finally getting a way to normalize audio output.

Some songs are barely audible, some are way too loud. Is there a program (either on Android/Windows/Ubuntu/OS X) that can take a set of MP3 files and normalize their output levels?