Retrospectively soundproofing drywall?

It there a substance I can put in my already constructed drywall to soundproof the room? I want a some sort of marble size insualtion that will drop down by gravity from a hole I create near the ceiling in the wall. I know I could fit specially produced soundproofing drywall sheets but this would mean rebuilding the room.
Paul Morgan Monifieth Scotland, November 2008


done
probably easier,quicker and cleaner 2 overboard with sound proof boards using an adhesive or dri wall adhesive or some sort of fixings ie long screws with washers on end or similar.

bert, December 2008
You may get caught out by horizontal bracing pieces between the vertical studs (Noggings or Dwangs in Scotland). Even if you manage to pour your marbles in at the top they will only fall as far as the upper noggin.

Peccavi, December 2008