I am looking to build a stage that will also be able to handle live music. If it's going to be at least 3 feet high, do I still need to fill it with sand? Should I just frame it and put down plywood? Would I put some kind of insulation under the plywood to absorb sound if there won't be any sand?

I plan on getting measurements and getting a 3D model of the stage on Monday or Tuesday.

I have no experience with a stage of that size but I think I would just line the inside surfaces of the cavity with thick insulation not try to fill it.

Just lay out a layer on the floor then put insulation in between the joists, similiar to insulating under the floor in crawl space construction.

I would construct the surface out of 3 layers of sheeting and put roofing felt between each layer.