Oh, I think you misunderstood me. It's not when you record. Different producers and sound technicians use different volume levels when they record music professionally.
For example, if I go from my Beach Boys - Pet Sounds album to my Black Crowes - A Tribute to a Work in Progress, I have to change volume levels because of what the standard was in different eras.
So, if you're finding the difference from artist to artist (or even album to album), it may just be that the sound was originally recorded differently. You could try that theory out by playing the music from the CDs in a normal CD player or a DVD player that isn't hooked up to the computer.
Like I said, one of the things some people really like about iTunes or Creative Media Source is that the software does a pretty good job of amplifying sound in the background for you so that you aren't constantly playing with your volume knob.
I hope that makes more sense.