Synonyms for Mutter:

n.

lodge, grunt, raise the roof, kick up a fuss/stink/argument, kvetch, sputter, take something further. complain, croon, under your breath, bleat, hiss, breathe, grouse, purr. happy. sounds. mutter (noun)
grumble, mumble, murmur, mussitation, murmuration, croak, muttering, murmuring, grumbling, gnarl, maunder, mussitate.

v.

grumble, mumble (verb)
croak, murmur, moan, grunt, whisper, sputter, complain, grouse, groan.
stammer (verb)
mumble, splutter, stammer, lisp, stutter.
voice (verb)
speak, groan, moan, enunciate, cry, intone, articulate, murmur, whisper, exclaim, emit, voice, utter, pronounce.

Other synonyms:

complain, kvetch, croon. bleat, grunt, grouse. breathe. hiss. lodge. hum
purr.
Other relevant words:
grumble, breathe, happy, grunt, lodge, sputter, hiss, gnarl, croak, croon, kvetch, mussitate, muttering, complain, bleat, grumbling, murmuring, grouse, murmuration, mussitation, sounds, maunder, purr.

Usage examples for mutter

  1. The seamen no longer hesitated to mutter their disapprobation aloud, and many a prophetic opinion was ventured concerning the consequences of such reckless risks. – The Red Rover by James Fenimore Cooper
  2. Susan could not turn over in the night without arousing Mary Lou, who would mutter a terrified " What is it- what is it?" – Saturday's Child by Kathleen Norris
  3. Once he roused a moment to mutter – Down the Mother Lode by Vivia Hemphill