The Collaborative International Dictionary of English v.0.48:

Mutter \Mut"ter\, v. i. [imp. & p. p. Muttered; p. pr. & vb.
   n. Muttering.] [Prob. of imitative origin; cf. L. muttire,
   mutire.]
   1. To utter words indistinctly or with a low voice and lips
      partly closed; esp., to utter indistinct complaints or
      angry expressions; to grumble; to growl.
      [1913 Webster]

            Wizards that peep, and that mutter.   --Is. viii.
                                                  19.
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            Meantime your filthy foreigner will stare,
            And mutter to himself.                --Dryden.
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   2. To sound with a low, rumbling noise.
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            Thick lightnings flash, the muttering thunder rolls.
                                                  --Pope.
      [1913 Webster]

WordNet (r) 3.0 (2006):

muttering
    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]
    2: a complaint uttered in a low and indistinct tone [syn:
       grumble, grumbling, murmur, murmuring, mutter,
       muttering]