Soundproofing was the process of making a room impervious to sound.
In 2372, Odo had the floor of his quarters reinforced and soundproofed by a work crew. Quark, who had previously complained about sound from Odo's quarters annoying him, noticed the work being done and thanked Odo for thinking of him. Odo then explained that the soundproofing was incidental and in no way done for the Ferengi's benefit. Quark figured he should have known better, but thanked Odo anyway. (DS9: "Crossfire")