If you want to get all techy the hot rod board will tell you to use micro glass beads suspended in latex paint on the floors before anything else. I got as far as buying the beads but never tried it.
How do you tell if it works or not? Find somebody with an Iphone and record the decible level inside when you go for a drive(my wife has the app on her Ipad)(my car runs at 92db at cruise, she wears earplugs when we go somewhere). Then do it again after you do the work. Only way I know to know for sure.