MP3Gain does not just do peak normalization, as many normalizers do.
Instead, it does some statistical analysis to determine how loud the file
actually sounds to the human ear. Also, the changes MP3Gain makes are
completely lossless. There is no quality lost in the change because the
program adjusts the mp3 file directly, without decoding and re-encoding.
From Chris Kuethe, <ckuethe@ualberta.ca> (Maintainer)