Don't listen to people that say it isn't worth it.
When I gutted my interior (the first time) to run all the wires for my system, I put down some sound deadening under the carpet and it made a huge difference. I used the generic crap from circuit city since I was crunched for time and that was the cheapest stuff that was actually made for sound deadening. I could have gotten some junk at home depot, but I didn't want to run the risk of it being goo like or stink in the summer. In retrospect I am glad I did it, I need to do the doors still.