Soundproofing workshed floor & roof
There are a few approaches to soundproofing.  One is a lot of mass, like you're describing with your walls.  Another is mechanical isolation, two solid "walls" with no openings between them, a gap between them, which can be air, and no mechanical connection other than where they are fastened to the building structure.  

You can use the same principle with the roof.  Make it a double wall of clear material such as this clear corrugated plastic panel, which will provide light in the daytime:



Those kinds of panels come in various lengths.  A standard size is often 8', which might be a hair short for your dimensions.  A 12' length can be easily cut to the size needed, and a little extra length will give you some protective overhang.  The panels are self-supporting, you won't need a bunch of roof struts or posts.  You also won't need to insulate the roof.

There are matching mounting strips that you would use at the high and low walls.  The inertness of the walls will dampen noise transmission at those points.  



Rip some angled strips of wood to put under the mounting strips to compensate for the roof pitch so that the mounting strips will be square with the panel.  

You can improve the isolation even more with a layer of rubber or dense foam weatherstripping, or even a strip of wool felt, between the mounting strip and the panel.  

To separate the two panel layers, you use two mounting strips back to back on top of the first panel (use long screws to go through everything and then into the top surface of the wall).  The back-to-back strips will be offset by half a corrugation so the panel corrugations are parallel between the layers (uniform gap).  

Between the high and low walls, there will be an air gap between the layers.  The combination of the plastic material and the isolated layers will be very effective at blocking noise.  It will also be very light weight.  

At the two sides, you would need to seal in the sound.  The side walls will need a wedge-shaped top piece to follow the roofline, and the height will need to match the panel at that point.  The walls will dampen sound for any connection there.  

The gap between the panels needs to be sealed on the sides, and both layers should be firmly fastened to the walls.  How to fasten and seal the bottom panel to the side wall will depend on whether the corrugation is facing down, up, or transitioning.  


If it's facing down, put a layer of rubber or dense foam weather stripping, or a strip of wool felt, between the panel and the top of the wall and screw the panel down.
If it's facing up, round a board to fit the corrugation (use the mounting strip as a guide).  Alternately, if a standard diameter PVC pipe matches the curvature of the panel corrugation, cut a section as a mounting surface.  Use silicone caulk to adhere it to the top of the wall and fasten through it.  
If the corrugation is transitioning, cut up a few mounting strips to create pieces that fit the gap between the panel and the top of the wall.  Position these every foot or so to serve as fastening supports.  Adhere the supports and the panel to the wall with  expanding exterior-grade foam, which will also seal the gap.  You can then screw the panel down at the support points.  


Between the two panel layers, see if the gap is the same as a standard diameter PVC tube.  The tube can be glued to the bottom panel and act as the spacer (fasten through the tube at the support points).  Another option is to use foam pipe insulation of a slightly larger diameter than the gap, with a PVC tube inside it to maintain its shape (fasten through the tube).  The insulation will seal the gap between the panels.

 

If you plan to use the shed at night, put fluorescent or LED lighting inside.  You can fasten it to the top of the walls.

If your goal is to block noise, take the walls all the way to the ground.  Then put whatever flooring you want inside.

Ventilation will be a problem because any opening will compromise the noise control.  If there will be a wall facing away from your house and not facing neighbors, you could put a small window or two in that wall (triple pane will provide better noise control).  You can open them to get some air and close them while you're working.  From your description of sand-filled walls, you would need to frame out the openings.

If you want a "sealed system", you may need to just open the door occasionally for fresh air.  You can condition the temperature, though.  If you will be working and using equipment, there might be little need for heat, even in the winter, because it will be well-insulated.  In the daytime, you will also have a lot of heat gain from sunshine through the roof.  

For cooling, you could use either a portable AC unit with an exhaust vent to the outside (put the hole on the least problematic wall, like the one facing away from your house, close to the ground; run a duct through the wall and use a self-closing exhaust vent to keep critters out), or a two part AC unit that needs only a small pass-through that can be easily sealed.