WordNet sense 1 (a low continuous indistinct sound; often accompanied by movement of the lips without the production of articulate speech):

murmur, mussitation, mutter, muttering, murmuration

WordNet sense 2 (a complaint uttered in a low and indistinct tone):

grumble, grumbling

WordNet sense 3 (making a low continuous indistinct sound):

susurrant, whispering

WordNet sense 4 ((of sound) relatively low in volume):

soft