1. a low, continuous sound, as of a brook, the wind, or trees, or of low, indistinct voices. 2. a mumbled or private expression of discontent. 3. Also called heart murmur. Medicine/Medical. a. an abnormal sound heard on listening to the heart, usually through a stethoscope, produced by the blood passing through deformed cardiac valves. b. in some persons a similar sound heard when blood passes through normal valves. 5. to make a low or indistinct sound, esp. continuously. 6. to speak in a low tone or indistinctly. 7. to complain in a low tone or in private. –verb (used with object) 8. to sound by murmurs. 9. to utter in a low tone: He murmured a threat as he left the room.