Most of the noise is the wind going around the canopy. Sound proofing on the fire wall helps a little as well as sound proofing on the floor. Good sound proofing material is dense and weighs a lot and doesn't do that much since most of the noise comes from wind around the canopy. My next plane will have firewall and floor sound proofing only. I was talking to a friend of mine who installed a Vetterman muffler and he said it did make a difference.
To sum it up, I think the best very best sound proofing is a good pair of ANR headsets. Some RV pilots seem to be happy with the earplug type of headsets too.
Steve Barnes "The Builders Coach"
If you don't want ANR and/or headsets then I urge the passive type inserts.
There are several brands. Mine are clairity aloft brand. The wife did not want the headset messing up her hair, (surprise.) We tried a pair at Arlington in 2007. Been loving them ever since. With the 40+ decibel reduction both of our RV's are nice to talk in, even at WOT w/ straight pipes. Our RV's had no real soundproofing and music is great sounding. ANR headsets work also. Just an option.