MP3Gain Express is a port of MP3Gain and AACGain to macOS and OSX. It allows you to analyze or apply volume gain to a list of MP3 or AAC files. Anyone who has used the Windows UI for MP3Gain should find this one familiar, as it was designed to look similar to that version.

projects.sappharad.com/mp3gain

OSX Version

github.com/Sappharad/mp3gainOSX