I have found that in all of my hatchbacks with folding seats interior noise increases (dun dun dun...)
DRAMATICALLY when the seats are flat. And even more that one time I had occasion to remove them. In the rain with the seats down you can hear individual droplets hitting the rear wheel arches, it's like you have exposed metal. Oh wait...
I also noted that car with more exposed plastic are louder, caves with rough walls are silent, and concrete hallways echo endlessly unless they have carpet.
Also I found this book that is explaining a link between interior surface and cloth, but the most fun tables are wiped out (cloth vs leather sound absorption). Data implies a 1dB reduction for a cloth seats over leather.
https://books.google.com.au/books?id...0level&f=false
So I would say:
1: Make sure all your seats are upright and the tray tables are stowed away (for minimum plastic area)
2: Fluffy seat covers are win (if you can strap them down properly)
3: If at all possible, cover large hard areas with something resembling carpet.
4: Flock your dash
This will provide the most compliant (for absorption) and roughest (for dispersion) surface over the most area on the inside of the vehicle