(1):

(n.) Repressed or obscure utterance.

(2):

(v. i.) To utter words indistinctly or with a low voice and lips partly closed; esp., to utter indistinct complaints or angry expressions; to grumble; to growl.

(3):

(v. t.) To utter with imperfect articulations, or with a low voice; as, to mutter threats.

(4):

(v. i.) To sound with a low, rumbling noise.