MP3Gain is an audio normalization software that helps minimize audio changes from file to file, especially very loud to very quiet. The program analyzes MP3 files and changes the volume without re-encoding or modifying quality. Batch operations are available and changes can be reversed.
wxMP3gain is a cross platform front-end for the MP3gain tool. While it has an official portable version, despite its name it’s not actually portable (registry-wise), thus linking to the PortableApps version.