I've hesitated to contribute to the 3d debate, probably because I'm not really bothered and because I never send them in anyway. That said, the clue does say 'sound from top of room' which persuades me that it is the 'closing off' bit which provides the answer i.e. sealing.
In other words it is the positioning of the word 'from' in the clue that convinces me which of the answers is the one to go for.