Downloading MacMP3Gain 2.1 for Mac from the developer's website was possible when we last checked. We cannot confirm if there is a free download of this app available. The program relates to Audio & Video Tools. We recommend checking the downloaded files with any free antivirus. The actual developer of this free Mac application is Apple Inc.
From the developer:
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.
You may want to check out more software for Mac, such as Xilisoft Video Converter Platinum or FlashVideo Converter, which might be related to MacMP3Gain.