Sound travels through anything.
If you tap one side of a lump of clay, a pulse will travel through it that you can feel on the other side.
I think they mean that clay doesn't
resonate.
eg, from http://www.bse.ph/download/EASE%20MODULES/SCIENCE/PHYSICS/Module%2016%20Sound.pdf …
However, when the frequency of forced vibration on an object matches the object's natural frequency, a large increase in amplitude occurs. This is called resonance …
Inelastic materials like clay do not resonate since they are incapable of vibrations. In order for something to resonate, it needs a force to pull it back to its starting position …