muttering voice; murmuring voice
Refers to a voice that is speaking in a low, indistinct manner, often as if talking to oneself. Related to the verb 呟く (つぶやく).
See also: つぶやく
I heard a small muttering voice from the next seat.
He always talks to himself in a murmuring voice.
Full kanji spelling; 呟 is not a jōyō kanji, so this form is less common.
Compound of つぶやき (muttering, the noun form of 呟く) + 声 (voice).