Aacgain normalizes the volume of digital music files using the replay gain algorithm. it works by modifying the global_gain fields in the mp4 samples. free-form metadata tags are added to the file to save undo information, making the normalization process reversable. it supports aac (mp4/m4a/quicktime) audio files in addition to mp3 files.

aacgain uses the same command-line user interface as mp3gain