mamrotać pod nosem

He looked at her and muttered something under his breath.

He saw that she was muttering something under her breath.

He muttered something under his breath but reached for the phone.

Haven't you ever stood next to a person who was talking to himself, muttering under his breath?

"She usually does," the press secretary muttered under his breath as he left the office.

He muttered something under his breath and made to turn away, but I held him.

In the end, he could only set off after her, muttering under his breath.

One of the dark men muttered something under his breath to those near him.

Muttering under her breath, she left to do as he had asked.

He muttered something under his breath and walked out of the room.