If you want a quieter interior with your windows up, tint your windows.
I recently got my windows tinted on my 2002 GT. I only wanted them to do the back hatch, the small windows behind the rear doors and the rear rolldown windows. However, they forgot and put the film on the front driver and passenger windows. I didn't pay for it, and I left it on.
The first thing I noticed is how much darker a "light smoke" film is than no film.
After ten minutes I got over the darker glass, and the second thing that I noticed is how much quieter it was in the car with the windows up. I guess the extra couple of mils of thickness on the glass and the softness of the film helps block the outside noise.
The stereo sounds better, too, because there isn't so much reflected sound from the glass; the softer film isn't as efficient in reflecting the sound.
I may have to take the film off of the front windows; any tint is against the guidelines in PA, and the cops are especially upset by tinted front windows. However, I'm tempted to replace the front tinted film with an untinted clear film, since it does such a nice job of quieting the car!