The seating will actually have a double decker platform for a couch, and then movie seats in front. I've just started to build out the first step of the platform...
Pressure treated wood used, and sill seal under to buffer any imperfections in the wood.
Put down a barrier so moisture so can't get into the insulation.
Insulation to buffer any sound reverberation since when done, the room will have pretty loud sounds system :-)