From Hydrogenaudio Knowledgebase
AACGain is a modification to the program mp3gain. It supports AAC (mp4/m4a/QuickTime) audio files in addition to mp3 files. Aacgain normalizes the volume of digital music files using the Replay Gain algorithm. It works by modifying the global_gain fields in the aac samples. Free-form metadata tags are added to the file to save undo information, making the normalization process reversible.
aacgain uses the same command-line user interface as mp3gain, and is compatible with the mp3gain Visual Basic GUI.