I have tried this before, but once your cat craps in your speaker, you can never get the smell out of your listening room! Even if you don't have a cat, clay litter is too light to do any good. Lead shot is the way to go.
Depends on the purpose of filling the cavity and what the manufacturer advises. If the purpose is to add mass or keep the cabinet from flexing, maybe not. I use unscented kitty litter in metal speaker stands to damp the ringing. No need to add any weight to my stands and it works very well for that purpose.
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