Meanings (ES + gloss)
murmurar
To utter words, especially complaints or angry expressions, indistinctly or with a low voice and lips partly closed; to say under one's breath.
The beggar muttered words of thanks, as passersby dropped coins in his cup.
You could hear the students mutter as they were served sodden spaghetti, yet again, in the cafeteria.
balbucear • rozarse
To speak softly and incoherently, or with imperfect articulations.
Meantime your filthy foreigner will stare, / And mutter to himself.
The asylum inmate muttered some doggerel about chains and pains to himself, over and over.