The Collaborative International Dictionary of English v.0.48:

Mussitation \Mus`si*ta"tion\, n. [L. mussitatio suppression of
   the voice, fr. mussitare to be silent, to murmur.]
   A speaking in a low tone; mumbling. [Obs.]
   [1913 Webster]

WordNet (r) 3.0 (2006):

mussitation
    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]