From BiblePortal Wikipedia

King James Dictionary [1]

Mut'Ter, L mutio, muttio, and musso,mussito allied perhaps to muse, which see.

1. To utter words with a low voice and compressed lips, with sullenness or in complaint to grumble to murmur.

Meantime your filthy foreigner will stare,

And mutter to himself.

2. To sound with a low rumbling noise.

Thick lightnings flash, the muttering thunder rolls.

MUT'TER, To utter with imperfect articulations, or with a low murmuring voice.

Your lips have spoken lies,your tongue hath muttered perverseness.  Isaiah 59

They in sleep will mutter their affairs.

MUT'TER, n. Murmur obscure utterance.

Webster's Dictionary [2]

(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.

Holman Bible Dictionary [3]

 Isaiah 8:19MediumSheol

International Standard Bible Encyclopedia [4]

mut´ẽr ( הגה , hāghāh   Isaiah 8:19;  Isaiah 59:3 ): An onomatopoetic word, used of the growling of a lion  Isaiah 31:4 , of the "mourning" of a dove  Isaiah 38:14 , or of the human voice, whether speaking inarticulately  Isaiah 16:7 or articulately (  Psalm 37:30 , "The mouth of the righteous talketh of wisdom" compare  Job 27:4;  Proverbs 8:7 , etc.). Hence, it is only the context that can give to hāghāh the meaning "mutter." No such meaning can be gathered from the context of  Isaiah 59:3 , and, in fact, the open shamelessness of the sinners seems to be in point. So the verse should be rendered, "Your lips have spoken lies, your tongue uttereth wickedness." In   Isaiah 8:19 hāghāh describes the tone of voice used by the necromancers in uttering their formulas, "that chirp and that mutter." That this tone was subdued and indistinct is quite probable. See Peep .

Cyclopedia of Biblical, Theological and Ecclesiastical Literature [5]

( מִהְגַּים , Mahgim, Mutterers), in  Isaiah 8:19, refers to the murmuring or indistinct enunciation of wizards and soothsayers in uttering their spells. (See Divination).