aacgain is a modification to Glen Sawyer's excellent mp3gain program, and
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, 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.

WWW: http://aacgain.altosdesign.com/