Use the word "mumble" when a person speaks softly or unclearly. It’s not a good idea to mumble, but some people do it when they don’t want another person to hear what they’re saying.
The teacher told the students to stop mumbling and finish their work.
A lot of people in the office were mumbling about their supervisor, saying she was being unfair. (People mumble when they complain privately about something.)
What are you mumbling about?
Or a person mumbles when speaking softly:
Edward mumbled a few words of apology to his mother before leaving the house.
The students who were mumbling during the presentation were told to stay after school.
Sometimes the act of mumbling is a speech deficiency which has to be corrected:
Christina has a bad habit of mumbling. She needs to learn how to speak clearly.
Jose mumbles when he speaks in English and in Spanish.
