This area of study, which didn't exist when I was at school, is concerned with wood, plastic, metal, glass etc. This has nothing to do with electrical resistance. They are resistant in the way that water isn't and the primary concern is with their strength, durability and suitability to different applications.
The easiest way of exploring these in relation to sound is to put examples into containers, themselves made of different materials, and shake them about. Is the sound of ten glass marbles being shaken in a glass jar different from the sound made by the same marbles being shaken in a tin can or plastic box? What if we shake some wooden beads in the same jar, can and box? What does this say about the density, resonance and other qualities of the materials involved? From this experience can we make predictions about the sound of other combinations of materials acting on each other? If you wear a blindfold can you tell what materials are involved in an experiment by the sound alone?
And finally, can you think of any reluctant KS3 children who might find this aspect of resistant materials sufficiently diverting to interest them in other areas of the topic?