I've used mp3gain for that. It's a command line utility.

You should be able to just say "apt-get install mp3gain" to install it.

MP3gain will work in Linux..................

The version of Mp3gain for Linux is called easymp3gain.

jjs@matisyahu:~$ apt-cache search mp3gain
mp3gain - Lossless mp3 normalizer with statistical analysis

I see no MP3gain, nor easymp3gain here, only mp3gain. How would one go
about finding the programs that you mentioned?

I found mp3gain in synaptic. Then I found easymp3gain at sourceforge.
There is a deb package here: http://sourceforge.net/projects/easymp3gain/

In my case, for mass mp3 files adjustment (hundreds in one folder) I use mp3gain
(CLI) and for single file I use Audacity (graphic interface).

easymp3gain can be found through the Snaptic Package Manager.

Not here - search for easymp3gain comes up empty. search for mp3gain
gets a hit.

jjs@einstein:~$ apt-cache search easymp3gain
jjs@einstein:~$ apt-cache search mp3gain
mp3gain - Lossless mp3 normalizer with statistical analysis

use this link and look at the files you can find it there.

http://sourceforge.net/projects/easymp3gain/files/