Not recommended if you have cats!
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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.
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.
My cat loves the idea, that way he can sharpen his claws on the grill after he's had a good constitutional! He enjoys this most while listening to Ted (guns & glory but no brains) Nugent's; "cat scratch fever". I have this set up myself in my high end rock speakers with the 15" woofer (Sirwin Vega)
best holiday wishes