The Collaborative International Dictionary of English v.0.48:

Murmur \Mur"mur\, n. [F. murmure: cf. L. murmur. CF. Murmur,
   v. i.]
   1. A low, confused, and indistinct sound, like that of
      running water.
      [1913 Webster]

   2. A complaint half suppressed, or uttered in a low,
      muttering voice. --Chaucer.
      [1913 Webster]

            Some discontents there are, some idle murmurs.
                                                  --Dryden.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Murmur \Mur"mur\, v. i. [imp. & p. p. Murmured; p. pr. & vb.
   n. Murmuring.] [F. murmurer, L. murmurare, murmurari, fr.
   murmur murmur; cf. Gr. ? to roar and boil, said of water,
   Skr. marmara a rustling sound; prob. of imitative origin.]
   1. To make a low continued noise, like the hum of bees, a
      stream of water, distant waves, or the wind in a forest.
      [1913 Webster]

            They murmured as doth a swarm of bees. --Chaucer.
      [1913 Webster]

   2. To utter complaints in a low, half-articulated voice; to
      feel or express dissatisfaction or discontent; to grumble;
      -- often with at or against. "His disciples murmured at
      it." --John vi. 61.
      [1913 Webster]

            And all the children of Israel murmured against
            Moses and against Aaron.              --Num. xiv. 2.
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            Neither murmur ye, as some of them also murmured.
                                                  --1 Cor. x.
                                                  10.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Murmur \Mur"mur\, v. t.
   To utter or give forth in low or indistinct words or sounds;
   as, to murmur tales. --Shak.
   [1913 Webster]

         The people murmured such things concerning him. --John
                                                  vii. 32.
   [1913 Webster]

WordNet (r) 3.0 (2006):

murmur
    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 schwa that is incidental to the pronunciation of a
       consonant [syn: murmur vowel, murmur]
    3: an abnormal sound of the heart; sometimes a sign of abnormal
       function of the heart valves [syn: heart murmur, cardiac
       murmur, murmur]
    4: a complaint uttered in a low and indistinct tone [syn:
       grumble, grumbling, murmur, murmuring, mutter,
       muttering]
    v 1: speak softly or indistinctly; "She murmured softly to the
         baby in her arms"
    2: make complaining remarks or noises under one's breath; "she
       grumbles when she feels overworked" [syn: murmur, mutter,
       grumble, croak, gnarl]

Moby Thesaurus II by Grady Ward, 1.0:

146 Moby Thesaurus words for "murmur":
   air a grievance, aspirate, aspiration, bark, bated breath, bawl,
   beef, bellow, bellyache, bitch, blare, blat, blubber, boom, bray,
   breath, breathe, breathy voice, buzz, buzzing, cackle, chant,
   chirp, clamor, complain, complaining, complaint, coo, crab, croak,
   crow, cry, dolorous tirade, drawl, drone, droning, exclaim,
   exhalation, flute, fret, fret and fume, fumble, fuss, gabble, gasp,
   gibber, gossip, grapevine, gripe, groan, grouch, grouse, grousing,
   growl, grumble, grumbling, grunt, hearsay, hiss, holler, howl, hum,
   humming, jabber, jeremiad, keen, kick, lament, lilt, little voice,
   lodge a complaint, low voice, maffle, maunder, moan, mouth, muddle,
   mumble, mumbling, murmuration, murmuring, mussitate, mutter,
   muttering, outcry, pant, pipe, plaint, planctus, purr,
   raise a howl, register a complaint, repine, roar, rumble, rumbling,
   rumor, scold, scream, screech, scuttlebutt, shriek, sibilate, sigh,
   sing, snap, snarl, snort, sob, soft voice, sough,
   speak incoherently, splutter, sputter, squall, squawk, squeal,
   stage whisper, still small voice, susurrate, susurration, susurrus,
   swallow, take on, talk, thunder, tirade, trumpet, twang, ululation,
   underbreath, undercurrent, undertone, wail, wail of woe, warble,
   whimper, whine, whisper, whispering, whistle, yap, yawp, yell,
   yelp, yowl