mutter, muttering, murmur, murmuring, murmuration, mussitation -- (a low continuous indistinct sound; often accompanied by movement of the lips without the production of articulate speech)
murmur vowel, murmur -- (a schwa that is incidental to the pronunciation of a consonant)
heart murmur, cardiac murmur, murmur -- (an abnormal sound of the heart; sometimes a sign of abnormal function of the heart valves)