Okay, so here's my thoughts on how you might construct a pretty massive wood-frame concrete-filled door. This is assuming a 32" x 80" door.
First, start with a layer of 1/4" ply then Green Glue then a layer of 3/4" MDF. On the bottom of this is an automatic door seal (I spec'ed a Zero International partial mortise).
Then, create a wood frame using 2x4s and 2x4s ripped in half (1-1/2" x 1-5/8", accounting for the width of the saw blade). This creates 8 cavities. Line the cavities with poly film and fill them with concrete. Let the concrete cure for at least a week.
Add another layer of poly and then a layer of 1/4" ply capped with a slightly smaller layer of 3/4" MDF.
Before getting to the specs, I wanted to cover a couple points. The slightly smaller layer of MDF is intended to create a "bank vault" effect, with staggered door seals. This double seal should do a fantastic job of blocking any air leakage. The Green Glue is important because mass without damping only covers half the battle. A quote from Ted White from another thread:
Originally Posted by Ted White
The issue is more than just the mass, as a damping of that mass would improve things as well. So the combo of mass + damping would yield great results.
This assumes things are sealed up very well. The seals are always the weak link.
The 1/4" ply is there simply because this door is already quite thick. As I write that, I realize that it would make more sense to use two layers of 1/2" MDF than 1/4" Ply + 3/4" MDF. It would be more dense and damping works best with layers that have the same mass and density, anyway.
So what are we looking at?
This door is 3-1/2" thick (roughly twice the width of a standard door) and is massive. How massive?
Assume that MDF is 49 lbs cu ft; Plywood is 32 lbs cu ft; concrete is 133 lbs ct ft; and Douglas fir studs are 31 lbs cu ft. There are 0.731 cu ft of plywood, 2.122 cu ft of MDF, 0.662 cu ft of wood, and 1.56 cu ft of concrete. Do the calculations and you have a door that weighs 355 lbs!
(Actually, it would be slightly more than that if you replace the plywood with MDF)
Is that enough?
Well, JH Brandt suggests that the door must have at least as much (and preferably) mass as your walls. If you assume that you have double walls with two layers of 5/8" drywall on each side and further assume that the insulation has negligible mass; the studs add only a little; and drywall is 2.2 lbs sq ft, then we have a mass of 8.8 lbs sq ft for the walls. Round up to 9 lbs, factoring in the studs.
Our door is 32" x 80", which is 17.8 sq ft. If we want our door to have at least as much mass as our walls, then it would need to be 17.8 x 9 = 160 lbs.
Our actual door is more than TWICE that massive. When you consider that it is also damped and fully sealed, then it's hard to imagine that this door wouldn't do a fantastic job of sound proofing.
Maybe too good, actually. Overkill is sometimes good, but exceeding the mass by that much might not be justified. That's a pretty extreme door.
I also have NO idea if it would actually work, since I haven't seen it done.