Well, if you put insulation in between the walls, I would guess you could get an STC of 65-70 or so, but that much sand in the walls is a bad idea.
To contain all that sand, you'd have to space your studs closer - 12" o.c. instead of 16", which would hurt acoustical performance - and to keep the walls from bursting you'd need to use 3/4" OSB/plywood sheathing screwed into the studs, on both sides, before any drywall. This gives you a solid panel inside the wall cavity, which is generally a no-no.
And having all that weight concentrated on a 3 1/2" wall plate, you'll probably crack your slab. That wouldn't be good...
Also, it's kind of futile, since you can't have sand in the ceiling. That's where all the noise is going to come from anyway. You've already got thick concrete/block walls down there; you hardly need the kind of overkill you're suggesting.
If you want good isolation, go with a double stud wall, insulated, with 2 layers of 5/8" drywall on each side. Mount the walls on a 1/4" layer of Mass-loaded vinyl (sold on this site) to reduce the flanking noise. If budget allows, use Green Glue between the sheets to absorb more bass.(
http://www.audioalloy.com) Seal up the wall real good and, voila, STC 65. Need more? Slap on another layer of 5/8" drywall per side and you might get 68. That should be good enough for an already quiet basement.
I think the 'room inside a room' idea is a good one, but do you have enough ceiling height to do that?