I use a command line utility called mp3gain, which is available in the repositories.

To use it, go into the directory where you store your music files, and then run this command:

find -name '*mp3' -exec mp3gain -r -k {} ;

It will go through every subdirectory, finding any file ending with .mp3, and normalize the gain so that they will also play back at a consistent volume.

Also note that mp3gain won't act on files that are already within the range you want, so the second time you run it and thereafter, it won't take as long as the first time. So you can just run this command as is every time you add new files, whether its just one new file, or a whole bunch.

If you happen to have .ogg files, there is a similar program called vorbisgain:

find -name '*ogg' -exec vorbisgain -r {} ;

There are options for how you normalize the gain for any one song in comparison to the rest of the music you have, but it's been so long since I've looked at them I can't really say much about it. I just know that I play my entire library as one huge randomized collection, and the above comand and its settings has worked for me for years.

If you happen to have .flac files, you can use metaflac:

find -name '*flac' -exec metaflac --add-replay-gain {} ;

Hope that helps.