I understand that sound is a wave through air. What’s going on with say, a bag of Doritos that makes every crinkle of the unfolding of the bag so loud? The loudness of a bag in the middle of the night seems out of proportion to the amount of air each crease can move as it unfolds.
A soft material will just change shape as you apply force to it. If you want a noise you have to make it change shape really fast.
A stiffer material will resist changing shape and then release the energy intermittently and noisily. Compare it to an earthquake where stress builds up over time until it’s violently released in an energetic single event.