Having used 1/2 plywood myself, I would recommend using 3/4", I think that alone would help deaden the sound. If you do use 1/2", get extra hold-down brackets to hold it down to the Mianne framework, I had some warpage in my plywood and needed a dozen or so extra brackets.
I'm using 3/8" anti-fatigue rubber matting as roadbed but still get some sound, and I don't use ballast (I did spray some of that textured stone paint on it though which gives the allusion of ballast).
You'll enjoy the build!!!
