Just my theory. He got one of those portable vocal booths (and no, it was not bigger on the inside), made a couple posts after that that he was doing all his listening in the booth, and then promptly stopped. Almost like he had bought JH Audio something or others.
And no, you don't need to seal everything, that was my point about attenuation. Find out what the walls already attenuate, find out how loudly you play, decide how quietly you want to be heard, what remains is what you need to attenuate by. No seal == still attenuate a lot, as long as you do things smartly like block the ductwork. In other words, concentrate on the problem areas.
And you don't need to hang the lead sheets from the walls, as long as you place them between the sound source and the largest conduits.