WordNet sense 1 (marked by absence of sound):

soundless, still

WordNet sense 2 (free of noise or uproar; or making little if any sound):

quiet

WordNet sense 3 (indicated by necessary connotation though not expressed directly):

tacit, understood, implied

WordNet sense 4 (implied though not directly expressed; inherent in the nature of something):

implicit, inexplicit

WordNet sense 5 (failing to speak or communicate etc when expected to):

mum

WordNet sense 6 (not inclined to talk or give information or express opinions):

uncommunicative

WordNet sense 7 (not made to sound):

unsounded

WordNet sense 8 (impossible to hear; imperceptible by the ear):

inaudible, unhearable

WordNet sense 9 (unable to speak because of hereditary deafness):

dumb, mute

WordNet sense 10 (without or deprived of the use of speech or words):

unarticulate