An introduction to computer audio
Replay Gain (also called volume leveling) is a standard proposed by David Robinson in 2001.
It allows media players to play all tracks with the same loudness.
It compensate for differences in mastering level.
Often you can choose between track gain (all tracks are normalized) or album gain (all albums are normalized but differences in loudness between tracks in an album remains).
Replay Gain does not alter the audio.
It is a tag written in the header of the audio file.
If your media player is set to use Replay Gain, the value of this tag will be used by the digital volume control.