A murmur can also be a soft, quiet thing that someone says. And if you murmur something, you say it in a soft, quiet way.
If a crowd is murmuring, or if you hear murmurs in a crowd, that means the people are talking in a quiet way, and although you can't hear exactly what they're saying, you can tell that they're worried, or upset, or complaining.
And if you hear murmuring from far away, or from another room, then you can't hear exactly what people are saying: you just know they're saying something, but it sounds low, quiet, and indistinct to you.
There was a musical group called The Murmurs, which is a fantastic name! It suggests that they sing softly and gently, with subdued emotions, which they did. It also suggests that their songs express anger or complaints, which they did. But the funny thing is, that's not why they called themselves The Murmurs. They picked that name because they thought it was funny and awkward how their teacher made them do vocal warm-ups that sounded like "Murrr, murrr!"