WordNet (r) 3.0 (2006):

unhearable
    adj 1: impossible to hear; imperceptible by the ear; "an
           inaudible conversation" [syn: inaudible, unhearable]
           [ant: audible, hearable]

Moby Thesaurus II by Grady Ward, 1.0:

21 Moby Thesaurus words for "unhearable":
   echoless, hush as death, hushed, inaudible, mousy, noiseless,
   quiescent, quiet, silent, soundless, still, still as death, stilly,
   subaudible, unarticulated, unpronounced, unsounded, unuttered,
   unvocalized, unvoiced, whist