他正在对着自己低声抱怨天气。

He was ___muttering to himself about the weather.

他咕哝着说这家餐馆太贵了。

He ___muttered something about the restaurant being too expensive.

“我不想去，”她压低声音嘟囔着。

"I don't want to go," she ___muttered under her breath.