intransitive/transitive to talk in a quiet voice that is difficult to hear, especially because you are annoyed or embarrassed, or are talking to yourselfHe muttered an apology and then left.
‘That’s a matter of opinion,’ she muttered under her breath.
mutter (something) about someone/something: Her husband muttered something about going out to find her.
mutter to yourself: He turned and went upstairs, muttering to himself.