in OST's build thread:
NOW... maybe I can think about carpeting, and the remainder of the interior....
I may be a bit more than competent when it comes to mechanical components, but when it comes to finishing details..... Lets just say I'm no expert when it comes to getting the interior just right.....
I've had an initial chat with a certified MECP specialist for some advice on different interior materials to knock down, minimize some of the sound levels BEFORE we assembly the interior. I'm not going for Lexus quiet... but I am going for quieter than expected sound levels...
Materials:
The two materials suggested were Dynamat, and Dynapad depending on where it is to be used.
Any other recommendations?
How much material will I need?
Where:
I'm HOPING to do the tops of both rear wheel wells(between the inner "structure" and the outer skin), as well as the inner side of the exterior rear 1/4 panels. Along with the outer sheet metal and wheel wells, I'll need to do the expanse of floor, from the rear tail lamps all the way forward to the front edge of the new compartment. Also, I'll need to insulate the floor to the firewall. I gave SOME thought to the roof... but I'm not into pulling down a perfectly good headliner
Have I missed something?
I've used Rammat, seems very similar to dynamat but less costly. I was pleased with results. There was some good info on their website.
I haven't used it but I think I've seen references to some sort of ceramic bead containing spray that might be worth considering.
I have no idea what kind of car you are working with or what your challenges are but here is a link to a thread that I had bookmarked for a Saturn Vue.
http://www.saturnfans.com/forums/showthread.php?t=141110&page=6
I bring you into that thread on page 6 because it has these pictures:

We built a new(old)service truck and because it was a 1984 GMC Topkick with a Cat diesel, I thought I would try to make it more comfortable for my guy to drive. We stripped the cab, and installed a new four speaker Bluetooth stereo and a Peterbilt airseat. We also spent 2 days dynamatting the whole interior including the back of the cab and the roof. We also added an inch of underlay from a pickapart minivan.
It is no quieter but the seat is nice.
Bang for the buck I usually got a bucket of vblok, but I am not sure if it's even still available.
These folks all make materials that will not only reduce sound, but unwanted heat as well. All of these companies also help to bring you this forum, as they support GRM and CMS. If you contact them, make sure to let them know you are a GRMer. Thanks!
Design Engineering: www.designengineering.com
Thermo Tec:
www.thermotec.com
Koolmat:
www.koolmat.com
Heatshield Products:
www.heatshieldproducts.com