The foam board may be a smoother surface, but when you screw the track down through it into the plywood, the vibrations will transmit into the plywood through the screws. It may reduce the sound level some, but not completely.


Larry

I agree on the liquid nails.   I am in the process of adding to my layout.  In that process I am removing a lot of the foam i put down years ago using liquid nails.   It is tough to get up sometimes.  So just be aware if you use it use what you need and little more just in case you ever decide to have a do over.