Filling speaker cavities with cat litter

what speaker cavities?  
hopefully you mean speaker stands.  

my old proac response 3.5's have a lower area of the cabinet (below the drivers) that allow the user to sand fill

stuart tyler said in the manual to use dry sand for mass loading

but for my skylan stands i use landscaping gravel or rice as the grains are larger and will not leak through the tiny seams

@ bnrlaw
Steel Shot is a better choice than cat litter!
https://www.ballisticproducts.com/Steel-Shot/products/70/

I didn't this question was asked in 2002, but I would suggest you use sand as it's less likely to absorb moisture and will be easier to remove.