I was playing around with dropping physical spheres onto nonphysical blocks in each of the different materials. Some of the built in sounds are interesting, but I noticed that the stone-on-stone collision didn't make any sound. Is there supposed to be a sound there, or is this a known issue that it's missing?
Editted to add:
After a few restarts, I was able to get the sound from the stone collisions. It's odd that stone was the only one not working before, but they're all working now.