I would say the easy place to start would be just to use db plaster board on all the walls and sealing and and ideally double board it staggering the joints if they are stud walls insulate behind in the cavity with Fiberglass wool if not build some wooden frames and stud it out.Its is basically just a very dense type of plater board something like 36kg a sheet and then get or make some extra thick doors with good seals.