Acetelene tanks have some sort of "matrix" in them and contain
acetone - this should make the tank heavier and might dampen the
"ring" of the empty tank?

You're right. They're filled with a porous concrete and this is
saturated with acetone, and the acetylene is dissolved into the
acetone. Makes things safe at the pressures we have to store the stuff
at. Free acetylene is dangerous above 15 psi.
You sure wouldn't want to fill anything else with acetylene.

Dan