"If you listen to music on CD much, you'll notice that some CDs
sound much louder than others, and I'm not talking about Ministry's
The Mind is a Terrible Thing to Taste versus Sarah McLachlan's
Touch. Two CDs of the same genre, when played on the same CD
player, at the same volume, can have drastically different playback
volume. This difference carries over when you rip the CD to MP3,
and can be really annoying when you're going from song to song on
your MP3 playlists on your computer or portable music player. One
way to iron out the differences is to use MP3Gain to adjust your
MP3s to have the the same volume.
"According to the MP3Gain site and documentation, MP3Gain does
an analysis of each MP3 and tries to figure out how loud it sounds
to the human ear. Then, after analyzing the file, it can save
information to the MP3's metadata in the APEv2 format to Analysis
and Undo tags about its analysis, and adjust the MP3's gain setting
directly so that the track is louder or softer..."