Software Description
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aacgain is a modification to Glen Sawyer's excellent mp3gain program. It supports AAC (mp4/m4a/QuickTime) audio files in addtion to mp3 files. If you are not familiar with mp3gain, stop reading this, and go to http://mp3gain.sourceforge.net. 

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 reversable. 


Website
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https://web.archive.org/web/20120717034554/http://altosdesign.com/aacgain/