To speak or utter indistinctly, as by lowering the voice or partially closing the mouth: mumble, murmur, whisper. See sounds
To complain in low indistinct tones: grumble, grunt, murmur. See happy, sounds
A low indistinct utterance of complaint: grumble, grunt, murmur. See happy, sounds
Another word for mutter
To speak as if to oneself
mumble, murmur, grunt, grumble, sputter, whisper, speak sotto voce (Italian), speak in an undertone, say to oneself, speak under one's breath, swallow one's words; see also mumble, utter.
grumble, moan, groan, grouse*; see complain 1. See syn. study at mumble.