The term 'make soundproof' can be solved by two crossword solutions: DEADEN (6 letters) and DEAFEN (6 letters). Soundproofing is the process of making a room or space acoustically insulated from outside noise. This can be done by using materials such as insulation, acoustic foam, and other sound-absorbing materials. DEADEN is a verb meaning to reduce the intensity of sound, while DEAFEN is a verb meaning to make someone unable to hear. Both words are used to describe the process of soundproofing a room or space.