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MacMP3Gain is an AppleScript Studio application which brings an Aqua GUI to the command line version of mp3gain, a utility that performs statistical analysis to determine how loud the MP3 file actually sounds to the human ear and performs lossless volume adjustments. MacMP3Gain is free and open source. You may download either the application itself or the entire project including the source code.
Important: MacMP3Gain modifies MP3 and unprotected AAC files. I would highly recommend a full backup of your MP3 library prior to running mp3gain on them.