mutter|muttered|muttering

他不停地自言自语。

He was ___muttering away to himself.

“埃米尔·左拉是最好的作家”，他嘀咕道。

"Emile Zola is the best writer", he ___muttered.

他嘀咕着说餐馆太贵了。

He ___muttered something about the restaurant being too expensive.