Other forms: murmurously
Anything that's murmurous sounds indistinct and quiet, like the soft, murmurous rustling of leaves in the trees on a breezy summer night.
This adjective comes from the noun murmur, which came to be defined as "softly spoken words" in the late 17th century but originally referred to an expression of discontent by grumbling. It shares an imitative root with the Sanskrit murmurah, "crackling fire." Murmurous things sound like people murmuring, or speaking so softly you can't necessarily hear what they're saying.