A couple of things to note. Lead is a hazard to humans if it's ingested or absorbed through skin. When inside a speaker stand, it really doesn't pose much of a concern. Environmental impact is a different story. Sand is a good alternative as is steel shot.
Cat litter is kinda nebulous because not all litter is the same. The most common form is diatomaceous earth (clay). Essentially absorptive dirt. Not particularly heavy, inert or good at vibration damping, it's recommendation has always puzzled me. Perhaps there's something I don't know, but when I've seen it recommended, it's just "cat litter".
Sand is small rocks. Bagged sand from home centers comes in a few forms. Play sand, masonry (sharp) sand, all purpose sand, etc. All of which are likely to have some moisture in them. More or less depending on how they're stored and handled. It's just not a concern when used for intended purposes.
If using sand, I think the best bet is silica sand. It's consistent in grain size, dry, and easy to come by. Used for sandblasting, masonry, ballast, filtering, etc. One could argue there's possibility of silicosis from silica dust, but in this application and exposure level that's just so little, it seems to me a red herring at best.
https://www.homedepot.com/p/100-lb-Silica-Sand-520-100-4095/202080636
