At anything much over 16K rpm, its gonna be loud. The prop noise is a monster even from a 9 " prop.
What exhaust are you running on it now ?? What did it measure at when you were checked for sound level (I assumed they checked it if you have a sound limit rule). If you are running the older rossi stock mini-pipe/muffler, those were notoriously loud.
Initially, might I suggest....
Loading the engine up with a 9x8 or 9x9 - get it down around 15K or less peak RPM on the ground.
Install one of the Jett-Stream mufflers - specifically the one designed for the SJ-60L. This is tuned for 14 to 17K. There is an adapter available for the rossi installation - simple bolt on configuration. The prop will unload in the air, bringing the engine up into the sweet spot for the muffler. I can't guarantee 96 db, but this will be quieter than what you have now.
Resulting combination will be very quick...... might even surprise you
The muffler has a fairly broad range, so if you want to drop to a smaller blade/pitch or go with a pylon prop (more RPM) the muffler will help the engine turn up even higher.
The port timing on the Rossi is fairly high, and dragging it down much lower than 14K peak will require a full size pipe set fairly long. Would be hard to install the configuration on a q-500 type plane ( I have tried... its not pretty)
Getting to 96db will be tough unless you drop the rpm and/or get a much more effective muffler.
For reference, my SJ-46 with a turbo-jett muffler..... 12x4 APC..... 14,000 rpm... installed within the cowl of my P-51 ... measured at 94 dB two weeks ago (3 meters, on grass, highest reading).