Anti-vibration balls for hifi racks

You could use steel shot instead of lead. Steel shot is used for waterfowl hunting to reduce environmental pollution. 

Sounds like good ideas, steel shot. 

A friend of mine was a DJ, he told me that they would put turntables on top of bicycle inner tubes, stretched across some wooden frames.  Hundreds of people dancing, and the turntables did not skip.

I guess I am a fan of isolation, rather than adding mass. 

I have a turntable hung from the ceiling, in my "man cave", off of long springs. It never skips. Looks cool too.

Kiln Dried Sand is the substance required. 

In the days of old, some would suggest a mixture of grain sizes was the best formula for Sand Fill. 

The Science for Kiln Dried Sand is that each grain is independent, and Kinetic Energy will attempt to transfer into each grain, resulting in energy transferal dissipating efficiently. 

The Frame for the Rack must be recognised to have a ring when Kinetic Energy is present. 

Sand seems like a good and inexpensive solution. I have no personal experience with following product, but it is marketed for this purpose: Atacama Atabite SMD-Z 7HD Inert Filler. I imagine there are others. 

Johnny Cat.  But just don't tell your cat.