Acoustic insulation should always be soft, e.g. Rockwell, so it muffles the sound. Rigid foam like Kingspan etc. are poor sound insulators, though excellent thermal insulators.
Hi Paul. You wrote " two 1,000 litre stores arriving Food grade containers £145 the pair delivered." How/where did you find the containers?
I am thinking along your lines, but need container(s) which will fit through the door.