Anti-vibration balls for hifi racks

Some audiophile friends of mine have had great success with squash balls.

Dunlop Sports Pro XX Squash Ball - Pro Box 

I believe he is asking about material to fill the legs.

Indeed, lead (or sand mixed with lead) was formerly a popular material for weighting and damping, and particularly for rack legs, until people became more conscious of the potential health risks. Even if you are not ingesting it or directly breathing it in, thermal changes can result in the release of toxic fumes (lead oxide).  Unless you are convinced there will be a reliably tight seal, lead is simply not a great material to be around, especially for children.

Sound Anchors uses sand in the legs of their steel racks.  I don't know for sure, but I would guess they use blasting sand, but not sure which grit.  A mixture of grit sizes will be denser than a single grit size but, I wouldn’t over-think it.  Medium grit would probably be suitable.

Sand should work. Use buckshot as intended. ;)

No matter what you use make sure it is dry. Any moisture and it will rust the inside and it would be a pain down the road.  If you go with steel balls or buckshot there is another option of shot peening material.  Steel shot for treating metal at sand blasting facilities.  It wears out and gets tossed.  Many different sizes although much smaller diameter than buckshot.  The used material should be almost free.  You could buy new of course buy I'm unsure of the current cost. Good luck John

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