To speak quietly and in a way that is not clear so that the words are difficult to understand. (Cambridge Dictionary)
Say something indistinctly and quietly, making it difficult for others to hear. (Oxford Dictionaries)
Bite or chew (something) with toothless gums or without making much use of the teeth. (Oxford Dictionaries)
To utter words in a low confused indistinct manner: Mutter. (Merriam – Webster)
To utter with a low inarticulate voice. (Merriam – Webster)
If you mumble, you speak very quietly and not at all clearly with the result that the words are difficult to understand. (Collins Dictionary)
To speak in a low indistinct manner, almost to an unintelligible extent; mutter. (Dictionary.com)
To chew ineffectively, as from loss of teeth: (Dictionary.com)
Why You Mumble (and Why You Need to Stop!)
Speak up! How ‘mumble acting’ is ruining TV and film.
Mumbling Isn’t a Sign of Laziness—It’s a Clever Data-Compression Trick.