The Collaborative International Dictionary of English v.0.48:

Murmuration \Mur`mur*a"tion\, n. [L. murmuratio.]
   The act of murmuring; a murmur. [Obs.] --Skelton.
   [1913 Webster]

WordNet (r) 3.0 (2006):

murmuration
    n 1: a low continuous indistinct sound; often accompanied by
         movement of the lips without the production of articulate
         speech [syn: mutter, muttering, murmur, murmuring,
         murmuration, mussitation]