I used MP3Gain to adjust the play volume of a whole directory of files for my wife. All in all it did a "pretty good job" (homegeneity was better in the processed directory than in the unprocessed directory)

But there were still some files that were at a lowere or higher relative volume than the rest. My wife pointed them out to me, and I used audacity to change the volume of those files separately from the rest. I really only had to change a few.. less than 5 as I recall. Out of a directory comntaining about 400 songs, I was fairly impressed by that.

But it's not perfect. And our original directory of of songs is 100% certain to be different than yours. As Notes says so often: "Your Mileage May Vary"

https://www.dropbox.com/sh/ey71h8ep047758h/AAC12jNM0Hz9Q8zsh5iDeqNHa?dl=0