You'll find the car actually has plenty of soundproofing already, you could add stuff like dynamat or trade equivalent (bitumen pads)but they don't half weigh a lot are a pain in the arse to fit involving lifting carpets and hairdryers and tar like smells for a few weeks.... Dynamat is horrendously expensive to boot.
Had a play with this on the doors and boot of the old 156 when I put in the ICE system, the main benefit was to keep the noise in the car, I didn't have the budget or the time to do the whole passenger compartment to deaden road noise - but you can do it....
there are some road surfaces that are noisy no matter what car I've ever been in a prime candidate is the part of the M6 just south of stafford - concrete theres a bit of the a50 between utoexeter and Stoke that is likewaise awful... It's even noticebale in my old mans e-Class.
As chris says some tyres make it worse - depends on the tread pattern the more grooves I think it is the more noise as a general rule?
Yes they do sound better with the stereo off - but when you are regularly doing 300mile trips a good sound system rapidly becomes necessary....
Alfa V6 - the ultimate aural pleasure
Simes - the Alfaowner formerly known as Alfa156man