On my layout I have cork on 2" foam over open grid. A short stretch of cork on 1 1/2" foam on 1/2" plywood. Spline on plywood. Spline on open grid. Track on cement roadbed on cement board and track running along a steel stud. The difference in volume does not change that much as a train moves from one material to the other. The one exception is the cement roadbed on cement board. In this case all you can hear is the wheels on the rails. This accounts for the majority of the noise we hear in my opinion. The only way to reduce that, would be to use plastic wheels, maybe. No thanks.
As far as all the others go the change from one material to the other is more of a change in frequency not so much a change in volume though it is there, but slight at best. I think more sound comes from the wheels on the rails than most people think. I also think the acoustics of the room play a larger role in noise than what the benchwork does.
I had my old 10’ x 5’ plywood pacific in the family room with ceramic tile floors and then had it in the carpeted dining room. There was a huge difference in the noise level yet it was the same layout.
On my current layout with many different materials used at various spots around the layout. You cannot tell what kind of benchwork the train is on once the sound is turned on. All you can hear is the engines.
Brent[C):-)]