I'm always slightly skeptical of any application that purports to modify the sound characteristics of an audio file without permanently altering the audio file. I got burned too many times in the past from bad Winamp plugins that permanently hosed audio files. MP3Gain is the real deal, tweaking the gain on your MP3 files to prevent massive discrepancies in volume between tracks without corrupting the file. The application will normalize any group of audio tracks, from albums to random collections of MP3s on your system using the replay gain algorithm. Read on for the geeky details about why MP3Gain works
As explained by the creators of MP3Gain: "The mp3 format stores the sound information in small chunks called 'frames'. Each frame represents a fraction of a second of sound. In each frame there is a 'global gain' field. This field is an 8-bit integer (so its value can be a whole number from 0 to 255). When an mp3 player decodes the sound in the frame, it uses the global gain field to multiply the decoded sound samples by 2(gain/4). So if you add 1 to this gain field in all the frames in the mp3, you effectively multiply the amplitude of the whole file by 2(1/4) = 119% = +1.5 dB. Likewise, if you subtract 1 from the global gain, you multiply the amplitude by 2(-1/4) = 84% = -1.5 dB."
There's also a version available to convert AAC audio files, as well as a Mac edition of the app. Bottom line, this free replay gain analysis tool is the best app I've ever tested for evening out the volume of MP3 files across a group of applications. Someone deserves credit for pointing me to the app, but I'm at a loss as to who it was. [Windows 9x/2k/XP $0.00]