Just do a Google for "Normalization". A very complicated process.
There are all kinds of factors involved. Type of music (Rock, Classical)
Recording method (Digital, Analog). Just leveling each song to say a
-6 db level doesn't guarantee equal volume perception.
Songs are recorded with differing levels of companding & limiting.
Some tools allow you to do "Batch" analysis, where each songs content
are compared and a "Average" loudness value is determined.
Most Broadcasting uses extensive audio processing. This is how
Radio stations achieve that "Consistent" volume level and songs sound
different than when played from a CD.
I use a program, Adobe Audition (CoolEdit Pro) that has a number
of features to "Normalize/Colorize" audio. There are lots of free tools
available that will allow you to correct your collection's levels.
Just be sure to keep your original songs before Normalizing, since
using Lossy formats can result in some quality degradation.
