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.
They murmured as doth a swarm of bees. --Chaucer.
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.
And all the children of Israel murmured against
Moses and against Aaron. --Num. xiv. 2.
Neither murmur ye, as some of them also murmured.
--1 Cor. x.
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