What you are probably experiencing is the exhaust sound getting "trapped" between the water and the platform. The composite platform doesn't help the situation much because it is solid front to back. There are many options available to try and help soften the blow when it comes to exhaust noise. Prior to my restoration that I am currently wrapping up on my comp, I had no mufflers whatsoever. From the exhaust manifolds it was straight pipes all the way to the transom and out the back of the boat. It wasn't loud except at certain speeds and throttle settings when it would reverberate off the water and the platform where having a conversation in the boat just wasn't going to happen. That speed is usually around 15 MPH to about 18 MPH or so if my memory serves me right. If you are truly wanting the quietest boat on the water that isn't a sail boat, look into FAE or Fresh Air Exhaust or make your own version of one if you have the ability as that is the only way I can think of making your boat as silent as possible.
'86 Comp TS6M - Reborn 2016
Riding a HO Sports CX Ski