Source : Webster's Revised Unabridged Dictionary (1913)

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.

Source : WordNet®

inaudibly
     adv : in an inaudible manner; "she spoke inaudibly low" [ant: {audibly}]