AACGain
AACGain is a modification to Glen Sawyer's excellent mp3gain program. It supports AAC (mp4/m4a/QuickTime) audio files in addtion 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 mp4 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. AACGain also integrates with iTunes using my iGain GUI.
