Not so long ago I was looking for a tool to equalise the volume of all my music, as it was getting annoying having to constantly adjust the volume when the tracks changed.

After some searching, I found MP3Gain. Using it is pretty simple; you can either run it individually per file, or combine it with something like find to modify many files at once.

I use two versions of the find command, as follows:

# The short version - when the directory only contains .mp3 files
find /path/to/music -exec mp3gain -k -T {} \;

# The long version - when a directory contains other files too
find /path/to/music -type f -name "*.mp3" -exec mp3gain -k -T {} \;

The most useful flag to this command (for me, at least) is -T, which modifies the original file rather than creating a copy.

Note that if you use any kind of library watcher with your music player, it may create duplicates if it’s open while you’re modifying the files. I would therefore recommend closing your music player until complete, or mp3gain your music in another directory before copying to your music directory.