The Collaborative International Dictionary of English v.0.48:

Somniloquism \Som*nil"o*quism\, n.
   The act or habit of talking in one's sleep; somniloquy.
   --Coleridge.
   [1913 Webster]

WordNet (r) 3.0 (2006):

somniloquism
    n 1: uttering speech while asleep [syn: sleep talking,
         somniloquy, somniloquism]