How much lead and silica sand in speaker stands?

Do not use either, lead is toxic and sand will make a mess and can seep out. Go to your home improvement store and by a bag of small pebbles and use them, they are stable, safe, easy to handle. Shake the stands after filling at different levels to make sure they are tightly packed. If you have to empty to sell or ship, just pour into a 5 gallon bucket. Make sure they are dry before filling.

Buy 5 pounds of sand and pack it in, a 4 pound stand... That will definitely be enough.. :-)

Pea gravel, is actually the best. Very, VERY, little settling, and sand you tamp, and it has to be dry..
Lead, well it is not to toxic, unless you breath it (heat it) or eat it.. It would be pretty expensive too. Lead ain’t cheap, it’s not gold but..Dry pea gravel is the best... Could hide your gold in them...Just sayin’

Regards

Small ball bearings or small pea size stones work best, fill them to the top as you want them to be dampened as much as possible. I then use some poster board putty formed into small golf ball size then pressed hard between the speaker and stand this couples the two nicely.

Steel shot is a little more environmentally friendly than lead.  Supposedly dust-free kitty litter is also an option.
I suspect some blend of the possibilities that fills all cavities might work best.

Sand / Silica dust is a breathing Hazzard ... use ⚠ caution ... generally the more the better ...