VorbisGain is a utility that uses a psychoacoustic method to correct
the volume of an Ogg Vorbis file to a predefined standardized
loudness.

It is meant as a replacement for the normalization that is commonly
used before encoding.  Although normalization will ensure that each
song has the same peak volume, this unfortunately does not say
anything about the apparent loudness of the music, with the end
result being that many normalized files still don't sound equally
loud.  VorbisGain uses psychoacoustics to address this deficiency.
Moreover, unlike normalization, it's a lossless procedure which
works by adding tags to the file.  Additionally, it will add hints
that can be used to prevent clipping on playback.  It is based upon
the ReplayGain technology.

The end result is that the file ends up with superior playback
quality compared to a non-VorbisGain'ed file.

It needs player support to work.  Non-supporting players will play
back the files without problems, but you'll miss out on the benefits.