For doors, i recommend applying a layer of damping material to the inside of the outer door skin. Next, plug any holes that you can on the inner door panels, with something like 1/4" ply or hardboard using liquid nails or silicone to apply. Then put a layer of damping material over the entire cabin side of the inner door panels. Next, put a layer of something like carpet foam between the door panel and the inner door frame. what you have is a multilayer sound deadening system that goes like thsi metal, dynamat, air, metal, dynamat, foam, plastic. Damping material reduces vibrations, but does little to reduce road noise. To reduce road noise, a change in impedance is required which is where the layers comes into play. You could probably go as far as trying to implement some sort of constrained layer damping but I don't know where to begin going into that, or using it effecitvely in a car.
