what is sound proof cement made out of?
I don't think it exists. Cement is typically used as a binding agent for concrete. Because it is very dense, it transmits sound vibrations thru it, and would not be sound proof. There are sound attenuating walls made of concrete, but they still allow some sound to pass thru. The only sound-proof place is inside an anechoic chamber!!!
Cement isn't really sound proof. If someone said something like I was trapped in a sound proof cement chamber It means that the chamber was sound proof, not the cement it self.
