Let iTunes automatically adjust sound volume to play songs at the same level
It has always peeved me that some songs will play louder than others. This is due to a multitude of reasons, from the source audio to the way it was ripped, but nonetheless it’s obnoxious when one song is quiet and the next is blasting. Stop messing with your speaker volume though, because iTunes will automatically adjust song volume level to be constant for you! It’s a setting that should be enabled by default if you ask me, but since it’s not here’s how to turn it on:
Enable iTunes automatic song volume adjustment
* From the iTunes menu, navigate down to ‘Preferences’
* Click on the ‘Playback’ tab at the top
* Select the checkbox next to ‘Sound Check’
* Now select ‘OK’
Now restart iTunes and the song volume will be consistent across songs, with iTunes automatically adjusting the levels for you. No more speaker tweaking! This works in Windows and Mac OS.

Handy tip! Most of my music plays at comparable levels, but there are a few songs that seem to be recorded at mere whispers compared to everything else.
It works well for pop / rock etc but is horrible for classical music split into one track per movement.
Does anybody know what exactly/technically the Sound Enhancer does? Or is it just a magical black box?
[...] like this on a per song basis is a good way to fix the way a single song plays, but you can also set iTunes to automatically adjust song volume levels and iTunes will attempt to play all music files at the same volume [...]
ScreenshotiVolume calculates the volume perceived by the human ear for each song of your iTunes music collection. Thereby iVolume gets the most out of the approved Replay Gain algorithm and adjusts your songs
http://www.mani.de/ivolume/