I second the high temp mastic, I bought some from mcmaster-carr for a Nissan Altima I used to have. Right after I bought it I noticed a lot of road noise, I covered the back deck, front doors (never got to the rears), and some additional along the trans tunnel. Road noise quieted a good bit. I read online that the biggest problem on that car is the tires, recommended Michelin, my budget never allowed for $100 tires on that little thing.
If I had to do it again I would try the "fat mat" I have seen on Ebay. Do the doors on the outer skin and inside. A layer or two on the trans tunnel and under the seats and the back wall.
If you live in Houston I have a couple of sheets left over I would donate if you let me know how it turns out. Stuff is heavy as hell and shipping from mcmaster carr was the bulk of the expense.