MP3Gain

In 2015 Debian and Ubuntu removed it from their repositories due to a lack of an active maintainer.

[3][4] MP3Gain first computes the desired gain (volume adjustment), either per track or per album, using the ReplayGain algorithm.

It then modifies the overall volume scale factor in each MP3 frame, and writes undo information as a tag (in APEv2, or ID3v2 format) making this a reversible process.

MP3Gain does not introduce any digital generation loss because it does not decode and re-encode the file.

A mod called AACGain[5][6][7][8] exists that can be used as super-set drop-in replacement[9] in most front-ends originally created for MP3Gain.