Product to fill up speaker/audio stands and racks

I prefer not to use sand. Unless you get a well dried silica sand, the moisture in it can aid in rotting your metal stands from the inside out.

Go to a reloading shop and get some lead shot. Cheap enough, extremely dead and doesn't make a mess. Just don't eat it.

I used playlot sand from Home Depot, loaded using a funnel. Theoretically, this could be messier than shot or rocks, but I like the better packing density.

I'm in the playsand camp too. It's cleaned and dried. Go to an auto parts place and get a good funnel with a long spout that you can manipulate around a bit. I measured out the sand in a separate container then used a plastic cup to move the sand from the bucket to the funnel. It took about 15-20 minutes per speaker but I hardly spilled a grain. I also like the resonant damping qualities.

I used playsand and shot. The sand to fill in the voids around the shot.
Worked very well.

I have also heard of some using Kitty Litter,,which was said to do just as well as sand....

Very Fine grade of steel shot, easily acquired at gun shops or steel blast, very resonant and mass heavy.

Another recommendation for lead shot. It has the most "dead" weight per volume, is easy to use and unless you spill some, no mess.