Do you hear that murmuring? Murmuring is a low sound that's one step above whispering. And anyone or anything that makes such a sound can also be described as murmuring, like the murmuring audience in the theater just before the play begins.
Some murmuring is discontented or complaining, similar to the word muttering: "Restless murmuring filled the room as the best man's wedding speech dragged on longer than anyone expected." We can trace murmuring all the way back to a root, mor-mor, that it shares with the Sanskrit murmurah, "crackling fire," and the Greek mormyrein, "to roar."