Nouns

(n)mutter, muttering, murmur, murmuring, murmuration, mussitationa low continuous indistinct sound; often accompanied by movement of the lips without the production of articulate speech
(n)murmur vowel, murmura schwa that is incidental to the pronunciation of a consonant
(n)heart murmur, cardiac murmur, murmuran abnormal sound of the heart; sometimes a sign of abnormal function of the heart valves
(n)grumble, grumbling, murmur, murmuring, mutter, mutteringa complaint uttered in a low and indistinct tone

Verbs

(v)murmurspeak softly or indistinctly“She murmured softly to the baby in her arms”
(v)murmur, mutter, grumble, croak, gnarlmake complaining remarks or noises under one's breath“she grumbles when she feels overworked”