There’s a preference setting in iTunes called Sound Check that can help equalize track volumes. It’s in iTunes/Preferences/Playback, and when you turn it on, Sound Check will go through your library and check the playback volume of each track.
What Sound Check does is analyze each track’s volume and then evens them out according to each other. Sound Check doesn’t alter the tracks; it measures the volume and then adds metadata to each in order for them to play back at about the same volume. If you haven’t used Sound Check before, it might take a while for iTunes to finish processing all your tracks.
You can also manually adjust the volume of tracks – like you can any track without using Sound Check – by highlighting the track and then going to the Get Info menu item. Click on the Options tab and you’ll see a volume adjusting control.rnrnSound Check will help control the volume when you’re playing tracks on your Mac or streaming with an Airport, but if you use Sound Check and want it to work on the tracks on your iPod, you also have to turn Sound Check on in your iPod. Turn Sound Check on or off in Settings in the main iPod menu. See Apple’s Knowledge Base article iTunes: About Sound Check.
Sound Check will not work on the copy-protected (DRM – Digital Rights Management) songs you got in the past from the Apple iTunes store. Because they are copy protected, nothing can be added to their metadata, so Sound Check will have no effect. But you can manually adjust the volume of tracks as above.
Ever since January, 2009, Apple has been selling non-DRM music, so any music you bought since then isn’t copy-protected and Sound Check will work on those tracks. See Apple’s Press Release: Changes Coming to the iTunes Store.
But that doesn’t mean all your music will change to non-DRM. You need to upgrade your tracks; prices vary. See iTunes Store: iTunes Plus Frequently Asked Questions (FAQ).
There are other utilities that do the same thing as Sound Check and claim to do a better job, as they use a better algorithm to calculate listening level and sound dynamics. iVolume gets good reviews, though it is $30 for a full copy, and $20 if you qualify for an upgrade. iVolume can work with single albums to adjust volume.
If you want to try an open source mp3 volume normalizer, check this article at Mac OS X Hints: Use a third-party MP3 volume normalization tool in iTunes – Mac OS X Hints.