Whispering: Speaking very softly, often confidentially or discreetly, with the lips barely moving and little or no vocalization.
Murmuring: Producing or characterized by a low, continuous, indistinct sound, resembling speech at a soft or mumbled level.
Mumbling: Speaking indistinctly or unclearly with the lips barely moving, resembling muttering or whispering but without necessarily conveying discontent or dissatisfaction.