mutter

n. 喃喃低语
v. 喃喃自语,作低沉声,出怨言

Actors often mutter to themselves when rehearsing their lines.

演员背台词的时候经常自己嘀嘀咕咕。

She kept on muttering to herself.

她老是一个人嘀嘀咕咕。