Sand will amplify the sound of your trains if it is glued down. I used powdery white sand from some dunes just to try it out, and can definitely hear a difference in the sound of the wheels on the track as a train goes through that area.
I use colored saw dust, sand, dirt & ballast material. It’s all glued down & looks real wherever you use it. Sound doesn’t seem to make a difference on my RR. I do use a good cork roadbed under most of my flextrack.
At the Cochise & Western Model Railroad Club we use Cat’s Pride cat litter as ballast on our HO scale layout. Cat’s Pride is kaolin clay and is marketed by Oil Dri Corporation. It can also be used to clean up oil spills and drips from driveways, etc., since that’s their primary business and cat litter is a sideline. Cat’s Pride does not have anything in it to make it swell up like other brands of litter.
I’ve ballasted hundreds of feet of track with sand, and it’s now my favorite ballast material. A couple of layouts ago I had about half the track ballasted with sand and the other half with a commercial ballast that was made from something like crushed walnut shells. I didn’t notice any difference in noise. Because of the prototypical variety of colors available, plus how easy it is to glue sand without it floating away, I’d be willing to accept a marginal increase in sound transmission as a tredeoff anyway.