Noun

• S: (n) mutter, muttering, murmur, murmuring, murmuration, mussitation (a low continuous indistinct sound; often accompanied by movement of the lips without the production of articulate speech)
• S: (n) grumble, grumbling, murmur, murmuring, mutter, muttering (a complaint uttered in a low and indistinct tone)

Verb

• S: (v) murmur (speak softly or indistinctly) "She murmured softly to the baby in her arms"
• direct troponym / full troponym
• direct hypernym / inherited hypernym / sister term
• derivationally related form
• sentence frame
• S: (v) murmur, mutter, grumble, croak, gnarl (make complaining remarks or noises under one's breath) "she grumbles when she feels overworked"

Adjective

• S: (adj) murmuring, susurrant, whispering (making a low continuous indistinct sound) "like murmuring waves"; "susurrant voices"