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.
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   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.
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            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.
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Moby Thesaurus II by Grady Ward, 1.0:

23 Moby Thesaurus words for "murmured":
   barely audible, decrescendo, dim, distant, faint, faint-voiced,
   feeble, gentle, half-heard, indistinct, low, pianissimo, piano,
   scarcely heard, soft, soft-sounding, soft-voiced, subaudible,
   subdued, unclear, weak, weak-voiced, whispered