mumble

/ˈmʌmbəl/
verb
  1. To speak quietly and unclearly, often with the mouth partly closed.
    • She mumbled an apology and quickly left the room.
    • He tends to mumble when he's tired, so it's hard to understand him.
    • Please don't mumble; speak up so everyone can hear.
noun
  1. Quiet, unclear speech.
    • His answer was just a mumble, so I asked him to repeat it.
    • The teacher ignored the mumble of complaints from the back of the class.
    • I could hear a mumble coming from the other room.