When listening to Ogg Vorbis (or MP3) files on your computer, you may have
noticed that some songs are louder than others. To a degree, this can be
what the artist or producer intended, at least when comparing songs from the
same CD. However, when (randomly) listening to songs from different CDs, the
sound level can vary quite much, so much that you need to adjust the volume
every now and then, to get a comfortable sound level. Wouldn't it be nice if
that could be handled automatically?

This is where VorbisGain comes in. It calculates a percieved sound level of
an Ogg Vorbis file using the ReplayGain algorithm. VorbisGain then stores in
the comments (tags) in the file a suggestion on how the volume should be
changed during playback, to get a uniform sound level. However, this only
solves half the problem; the player application needs to act on that
suggestion for it to be any useful.

WWW: http://sjeng.sourceforge.net/vorbisgain.html