murmur

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

make complaining remarks or noises under one's breath

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

speak softly or indistinctly

“She murmured softly to the baby in her arms”