Same word as quieten:

choke off, clam, clam up, close up, cool it, cut off, cut short, dampen, deaden, decrease the volume, dry up, dull, dumb, dummy up, extinguish, gag, hold one's tongue, hush, hush one's mouth, hush-hush, keep it down, lull, muffle, mute, muzzle, overawe, pipe down, quash, quell, quiet, quiet down, say nothing, shush, shut up, sit on, soft-pedal, squelch, stifle, still, strike dumb, subdue, suppress, tongue-tie