> the biggest problem with doors is not so much the mass of the door but the seal...
Listen to Pucho.
A door has to be incredibly lame to leak as much as its cracks.
I think you need much more than a spring closer and sili-foam to justify any more than a standard solid-core door.
And I have real doubt about 250 pounds sand in a 2x2 and chitboard structure; as Rod says, stability is vital and sand is not stable. Aside from sagging structures more solid than your wood, it settles. Fill it, pack it, ram it, set the top-wood on... and next time you slam the door the top 2 inches is hollow-core again.
It is also folly to have a pretty decent solid door, as in the building where I work, and then have a contraction crack where the wall hits the roof. Don't haul door-sand until you cover *every* detail in the roo's many leakages. Weak link in chain, yada yada.
How much attenuation do you really need? A lot of good music has been done literally in a basement with the door open. (OTOH, there is a Belafonte track done in a "professional" studio, and you can hear the thunderstorm outside.) The door may make you happy. The music may make many people happy.
> Ive seen 'fire rated' heavy doors used, filled with gypsum I believe.
There are several grades of "fire rated door", used for different occupancies. My school has some just plain wood fire doors, but rated to be solid and not made with real flammable glues. They also have steel doors. The wood door will hold back office-type fire for many-many minutes; try it. Long enough to get everybody out. The steel door will continue to function longer, may assist (or impede) fire-fighters. Where you want a door (and walls) to resist a fully enflamed building and protect contents, gypsum-filled is an option because gypsum will absorb near as much heat as water. And if you've camped, you know how much wood it takes to hot-up a pot of water.