I understand your delima completely...as for the closet doors, I always took them off, placed them in the same closet they were from and purchased some curtain rods that are spring loaded, and put them at the top of the closet door opening and then purchased some material to cover the closet. You don't even have to sew the material if you don't want to. There is an amazing product called stitch witchery, it is a tape like material that bonds two peices of material together when you iron it! You can even to tabs on the top of the curtains to make them slide easier or use decorative clips for faster access..
As far as the hollow core doors go..there really isn't any solution that I have found other than having a small fan going in the bedroom as background (white) noise. We usually did that combined with music playing..and that seemed to help! You might want to try painting the doors with some sound proofing paint...they have it, I just don't know how expensive, and if you can tint it. You could also try getting some corkboard and maybe applying it to the back of the doors it is what is used in flooring, but I don't see why you couldn't use it on the back of the door, and it is light weight!!
Hope this helps!