Hello Scott. Well thats hard to say. First off you have an attic that leads directly to the roof of the house. Second, the floor is tied to the house structure itself. Third, you have the dryrot problem around the exterior wall/floor framing, although I don't know how this would affect the structural elements under the wall that seperates the laundry/entry from the studio. There may be dryrot in the ends of the joists that support this wall, but untill you open the floor, you won't know. Even if you were to remove the OSB from this wall insulated and double drywalled it, and put a solid core door with seals, it still may transmit through the floor and or cieling. Impossible to say. Thats why this stuff is usually an ALL or NOTHING proposition. All I could say is, if you have the money and time to gamble, go ahead and try it. It may or may not do the trick.
However, let me advise you against adding any wieght to this ceiling as it exists. From what I've seen, thats a very risky proposition.

Had you decided to carry on with your original plan to make this space a robust isolated room, I would have told you that very extensive cieling/roof structural repairs and additional framing were in order. Even then, with the existing dryrot, the additional loads on the studs/foundation/joists may have disasterous effects. Thats why the floor joist/stone foundation/skirt issue would have to be addressed first. Frankly, your decision to forgo this aspect is wise, as it would have entailed DRASTIC demo and repairs from the getgo.
Anyway, you'll know more after you open this wall. Better open the floor though and check those joists closest to this wall support, as they carry the load. If you find dryrot and or any collapse of the stones, personally, I'd forget it, unless you want to repair all that crap first should it occur this way.
Well, thats my .02. Good luck Scott.
fitZ