She is muttering to herself.
There is muttering in the street about the need for a change of government.
I wish you would stop muttering.
If you have something to say, just say it.
muttering to yourself won’t solve anything.
Tell me clearly what is bothering you.
The old man was muttering to himself as he looked through his drawers for a pair of socks.
The students were muttering something about having too much homework as they left the classroom.
One student was muttering to himself all through the test, and it was distracting some of the others.
The students were muttering to themselves, reviewing the vocabulary in their heads one last time before the test.
White America gets to choose to turn a cheek, charitably muttering an obligatory “it’s such a shame.”