Self confidence and being prepared before speaking will help eliminate mumbling. Usually we mumble when we are not sure of what to say or have fear speaking in public. Sometimes we mumble when we do not know the subject we speak about very well. We seem to stumble, stutter, mumble around trying to think of what to say. Best to not speak until you have in your head what you are going to say. Educate yourself before speaking, or work on self confidence, or work on your speech so you are able to speak clearly.
This may apply to you and it may not, but is food for thought.