noun
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: 呟く
隣の席から小さなつぶやき声が聞こえた。
I heard a small muttering voice from the next seat.
彼はいつもつぶやき声で独り言を言う。
He always talks to himself in a murmuring voice.
呟く is the verb 'to mutter' or 'to murmur'; つぶやき声 is the noun for the voice or sound produced by that action.
Compound of つぶやき (muttering, the noun form of 呟く) + 声 (voice).