MP3Gain analyzes and adjusts mp3 files so that they have the same volume. It
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.