The Collaborative International Dictionary of English v.0.48:

Obmutescence \Ob`mu*tes"cence\ ([o^]b`m[-u]*t[e^]s"sens), n. [L.
   obmutescens, p. pr of obmutescere to become dumb; ob (see
   Ob-) + mutescere to grow dumb, fr. mutus dumb.]
   1. A becoming dumb; loss of speech. --Sir T. Browne.
      [1913 Webster]

   2. A keeping silent or mute. --Paley.
      [1913 Webster]