mutter

nouns

a low continuous indistinct sound; often accompanied by movement of the lips without the production of articulate speech
murmur, murmuration, murmuring, mussitation, mutter, muttering
is derived from
mutterer
a complaint uttered in a low and indistinct tone
grumble, grumbling, murmur, murmuring, mutter, muttering

verbs

talk indistinctly; usually in a low voice
maunder, mumble, mussitate, mutter
is derived from
mutterer
is derived from
muttering
make complaining remarks or noises under one's breath
"she grumbles when she feels overworked"
croak, gnarl, grumble, murmur, mutter
is derived from
muttering