When you speak in a voice so low and soft it can barely be heard, it's a murmuration. A speaker might get distracted if she hears the sound of the audience's murmuration.
To talk very quietly is to murmur, and one name for the sound this makes is a murmuration. This word is generally used in novels and poems, often describing something that resembles the sound of people murmuring, like the murmuration of rustling leaves in the trees. Murmuration is also a term of venery — a collective noun for a group of animals. If you see an enormous, swooping flock of starlings in the sky, you can call it a murmuration.