murmur

a low continuous indistinct sound; often accompanied by movement of the lips without the production of articulate speech

a schwa that is incidental to the pronunciation of a consonant

an abnormal sound of the heart; sometimes a sign of abnormal function of the heart valves

a complaint uttered in a low and indistinct tone

speak softly or indistinctly; "She murmured softly to the baby in her arms"

make complaining remarks or noises under one's breath; "she grumbles when she feels overworked"