The Collaborative International Dictionary of English v.0.48:

Mumble \Mum"ble\, v. t. [imp. & p. p. Mumbled; p. pr. & vb. n.
   Mumbling.] [OE. momelen; cf. D. mompelen, mommelen, G.
   mummelen, Sw. mumla, Dan. mumle. Cf. Mum, a., Mumm,
   Mump, v.]
   1. To speak with the lips partly closed, so as to render the
      sounds inarticulate and imperfect; to utter words in a
      grumbling indistinct manner, indicating discontent or
      displeasure; to mutter.
      [1913 Webster]

            Peace, you mumbling fool.             --Shak.
      [1913 Webster]

            A wrinkled hag, with age grown double,
            Picking dry sticks, and mumbling to herself.
                                                  --Otway.
      [1913 Webster]

   2. To chew something gently with closed lips.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Mumbling \Mum"bling\, a.
   Low; indistinct; inarticulate. -- Mum"bling*ly, adv.
   [1913 Webster]

WordNet (r) 3.0 (2006):

mumbling
    n 1: indistinct enunciation
    2: ineffectual chewing (as if without teeth) [syn: mumbling,
       gumming]

Moby Thesaurus II by Grady Ward, 1.0:

38 Moby Thesaurus words for "mumbling":
   aspiration, bated breath, breath, breathy voice, drone, droning,
   exhalation, gabble, gibber, gibbering, jabber, jibber,
   little voice, low voice, maundering, mouthing, mumble, murmur,
   murmuration, murmuring, murmurish, murmurous, mutter, muttering,
   rustling, sigh, soft voice, stage whisper, still small voice,
   susurrant, susurration, susurrous, susurrus, underbreath,
   undertone, whisper, whispering, whispery