I had a similar issue: I have hearing problems and I'm sensitive to higher-pitched sounds and loud noise in general. I bought some vinyl-backed material from McMaster-Carr and made a custom sound pad which is actually sort of similar to what they used in 911's. It's black with a pinhole pattern that is similar to that of headliners. If someone didn't know any better, they could be convinced it was factory. In any case, it made a YUGE difference in the sound volume in the cabin. I wish I had taken sound measurements before so I could offer concrete numbers. The foam soaks up higher frequency sounds than the dynamat/jute/tarboard/undercoating.
This is the material:
https://www.mcmaster.com/#9385K23I only made a layer from the "step" in the firewall up to the top. In hindsight, I would have made the slots for the hinge mounts shorter. Using the 1" thick material brought the top level out flush with the bottom. I'm still on the fence as to whether or not I am going to do the sides; it will help, but not as much as covering the section of the firewall did.
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