I applied sound deadener to the trunk area using
FatMat Trunk Kit ($50) and I'm very happy with the results. I am not an audiophile and rarely turn music on but it noticeably reduced the drone from the exhaust and road noise coming up through the rear of the car.
Took about 2hrs to install and there was a faint butyl-like smell in the trunk for the first 48hrs but went away after that, even on hot sunny days.