The following review written by one of the Software Informer contributors applies to version 3.5
iVolume is a volume adjustment tool for Apple iTunes. It uses a modern sound analysis algorithm to scan your audio files added to iTunes and detect each song's perceived loudness.
Then, it can increase or decrease the song's volume so that all songs have about the same loudness. Thus, iVolume attempts to take some work off your hand when listening to music: if you carry out such analysis and fix your whole iTunes library, you won't have to change volume settings when listening to music. It will all play smoothly and level.
iVolume detects your iTunes playlist and lists your songs. There, you can let the application do its magic, or you can change the volume output manually, by moving a bar to the left or right. Additionally, you can choose to combine songs, to create a gapless playlist. iVolume is compatible with Genius playlists, and it can detect songs added to your Genius playlists as well.
The graphical interface is very nice-looking. The application is cross-platform and takes advantage of multi-core systems as well. I found that processing the two sample songs included with iTunes took less than 5 seconds. On another note, I think that heavy-duty audiophiles will probably benefit more than the regular users by using iVolume. The built-in volume adjustment feature in iTunes usually seems to suffice to the untrained ear.