Filling speaker cavities with cat litter

I don't know,, that stuff gets pretty dusty.. I can see it know,,, guy buys a mint pr of speakers but something seems wrong.. He pulls the driver out and its full of clay dust not to mention what else he may find... Whats that smell?? Too funny

Since this stuff is designed to absorb moisture, I would be concerned that it would be pretty sensitive to humidity.

On a more serious note there may be much better choices of filler. Check with Industrial Abrasives suppliers for some safer (than lead) alternatives such as glass beads or steel shot. Both are rather moisture resistant and come in numerous (small) sizes. Is there some reason you don't want to use playground sand? Other than the downside of it wanting to attrack moisture, it's very inexpensive and usually available at most home remodeling centers.

Rather than "playground" sand look for an industrial grade variety. I went with a Mesh 30 grade which was essentially the same price as the playground stuff. The difference is it's been screened to remove the larger pieces (a.k.a. pebbles), so is a bit denser by volume. The finer consistency includes a bit higher dust content on a percentage basis which should be consider when handling. It hasn't proven to be a problem otherwise.