The Collaborative International Dictionary of English v.0.48:

Inaudible \In*au"di*ble\, a. [L. inaudibilis; pref. in- not +
   audire to hear: cf. F. unaudible. See In- not, and
   Audible.]
   Not audible; incapable of being heard; silent. --
   In*au"di*ble*ness, n. -- In*au"di*bly, adv.
   [1913 Webster]

WordNet (r) 3.0 (2006):

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