Originally Posted by sglynn
Yes I put sound DAMPING material on floor and sound ABSORBING material also. There is no such thing as dampening.
Not exactly true; I got dampened just today when I got caught in the rain.
Re the OP's question, like Bob I put sound absorbing material behind every panel from firewall to baggage bulkhead. It does make the plane noticeably quieter(and heavier), but in retrospect I would not do it again. If you value your hearing, the cabin is still far too noisy to fly without good anr headsets.
I recommend leaving out sound absorbing/damping material and enjoy the weight savings.