The Collaborative International Dictionary of English v.0.48:

Earwitness \Ear"wit`ness\, n.
   A witness by means of his ears; one who is within hearing and
   does hear; a hearer. --Fuller.
   [1913 Webster]

Bouvier's Law Dictionary, Revised 6th Ed (1856):

EAR-WITNESS. One who attests to things he has heard himself.