Link for more information:
iVolume improves the built-in 'Sound Check' function of iTunes to have your songs actually play at the same volume level.
iTunes uses a very fast algorithm to quickly analyze the volume of its songs. It's fast, but it's not very accurate. iVolume corrects the volume information that iTunes has calculated with better values. Therefore an algorithm is used that is oriented on the perception of the human ears. The result is a huge improvement of the 'Sound Check' function. You never have to readjust the main volume again when playing back your songs in random order. This is perfect e. g. for music on partys or hearing songs in random order with the iPod.
iVolume does not re-encode your audio files, so there is no loss in quality. Technically it looks for the tag named 'iTunNORM' that iTunes has put into your songs and iVolume just replaces the embedded value describing the volume of the song with a better one.
iVolume works closely with iTunes. Instead of reinventing the wheel or complicated handling with files at the filesystem level, iTunes is used as the interface to select the songs you want to adjust with iVolume. After selecting songs in iTunes, just switch over to iVolume and hit the 'Adjust' button to make the selected songs to the same volume level.