The Collaborative International Dictionary of English v.0.48:

microphonous \mi*croph"o*nous\ (m[-i]*kr[o^]f"[-o]*n[u^]s or
   m[i^]*kr[o^]f"[-o]*n[u^]s), a.
   Serving to augment the intensity of weak sounds;
   microcoustic.
   [1913 Webster]