Building a layout

I would cover the entire tabletop with Homasote or 2 inch thick extruded blue or pink foam insulation. Cover the seams with regular 3 inch wide masking tape. Finally, paint the entire surface with a flat brown earth colored latex paint. This will seal the tape and the surface while providing a dirt base color under subsequent applications of scenery material.

If the Benchwork is uneven, Shim it level.

Many have put a bolt and T-nut into the bottom of the legs to allow adjustment.

Then add sound supression ala Homosote, Quietbrace or Celotex Soundstop.

Those increasingly suppress sound in that order from my research.

I use 3" of the pink foam on top to allow digging Ravines and such in the terrain.

I had an uneven joint in the plywood.  I hid it in the scenery.  First I made my terrain with cardboard strip.  Covered it and the joint in the plywood with blue paper shop towels coated in fast set drywall mud.  Added rocks and ground cover.  It's gone.  It was a pretty ugly seam until I covered it.  It is the area between the tracks in the Photo.

I used a flooring compound to feather seams and add small grade details. Flooring compounds are a bit more difficult to work with (sand) compared to drywall joint compounds or hydrocal. 

Still best to have the bench work as level as possible.

Here is a slideshow of how a diorama was developed using flooring compound.  Click on the underlined phrase to access.

Or access this photo-bucket file.