EGD
I spent 30 years on the C130K and many suggestions were made to try to mitigate the sound levels 'down the back'. I was the liaison officer for a university team that was commissioned to conduct noise level trials. Removing the litter brackets and finding a stowage for them on the a/c and other measures had long been suggested by us . All of these suggestions were ignored probably by those senior enough not to have to travel in the cargo compartment. You are correct that the green covering on the walls is a protective blanket and has no sound attenuating properties at all. However if heated to around 800 degrees C it gives off Phosgene gas and this was a major factor in the loss of life in the Belgian AF crash at Eindhoven.