From wikipedia:Replay Gain:
- Replay Gain is a proposed standard published in 2001 to normalize the perceived loudness of computer audio formats such as MP3 and Ogg Vorbis. It works on a track/album basis, and is now supported in a growing number of players. Although the standard is formally known as "Replay Gain," it is also commonly known as "ReplayGain" or "replaygain." It is sometimes abbreviated "RG."
MPD has support for replay gain under some restrictions. MP3 replay gain tags need to stored as ID3v2, while most replay gain analyzers write to APEv2.
Replay gain is configured using the replaygain configuration option.