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

murmur, murmuring, mussitation, muttering, murmuration

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

grumble, grumbling

WordNet sense 3 (talk indistinctly; usually in a low voice):

maunder, mumble, mussitate

WordNet sense 4 (make complaining remarks or noises under one's breath):

croak, gnarl