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.
      [1913 Webster]

            Meantime your filthy foreigner will stare,
            And mutter to himself.                --Dryden.
      [1913 Webster]

   2. To sound with a low, rumbling noise.
      [1913 Webster]

            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]

Moby Thesaurus II by Grady Ward, 1.0:

79 Moby Thesaurus words for "muttering":
   aspiration, bated breath, beefing, bellyaching, bitching, breath,
   breathy voice, complaining, complaintful, crabbing, crabby, cranky,
   croaking, disappointed, discontented, disgruntled, displeased,
   dissatisfied, drone, droning, envious, exhalation, faultfinding,
   gabble, gibber, gibbering, griping, grouchy, grousing, growling,
   grumbling, jabber, jibber, little voice, low voice, malcontent,
   malcontented, maundering, mouthing, mumble, mumbling, murmur,
   murmuration, murmuring, murmurish, murmurous, mutter, out of humor,
   peevish, petulant, querulant, querulous, rebellious, resentful,
   restive, restless, rustling, sigh, soft voice, stage whisper,
   still small voice, sulky, susurrant, susurration, susurrous,
   susurrus, unaccepting, unaccommodating, underbreath, undertone,
   uneasy, unfulfilled, ungratified, unhappy, unsatisfied, whiny,
   whisper, whispering, whispery