With iVolume you can bring all your iTunes songs to the same loudness.

Therefore no simple peak analysis is made. Instead, the real from the human ears perceived loudness is calculated with an appropriate algorithm. The results of iVolume are much better than those of the built-in "Sound Check" function in iTunes.

If you want to learn more about the used methods you can find it at the following address:
http://replaygain.hydrogenaudio.org/

iVolume does not change any data of your audio files, so there is no loss in quality. iVolume simply adjusts the "Volume Adjustment" slider that you can find in the information dialog of iTunes for every song.

New in v3.5

• Compatibility: iVolume now fully supports iTunes 9 and Snow Leopard.
• Good bye 'No iTunNORM Tag'! You never have to deal with this annoying warning from the past again. Just forget about it. iVolume now fixes that automatically in a smart way.
• Reliability: Other errors like 'File Not Found' are also reduced to a minimum.
• Performance: Overall performance and stability enhancements.
• Maintenance: Various other improvements and bug-fixes under the hood.