AACGain is a modification to Glen Sawyer's excellent MP3Gain program. 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 reversable. AACGain uses the same command-line user interface as MP3Gain.