WordNet sense 1 (deaden (a sound or noise), especially by wrapping):

damp, dull, muffle, mute, tone down

WordNet sense 2 (smother or suppress):

stifle

WordNet sense 3 (make vague or obscure or make (an image) less visible):

deaden

WordNet sense 4 (lessen in force or effect):

break, soften, weaken

WordNet sense 5 (make moist):

moisten, wash