I don't mumble, but I don't project enough and my tongue moves faster than both my thoughts and my lips.
Then I just end up sounding like a gnat flying around a pile of dung. I'm trying to learn how to articulate, however.
Often times, when someone mumbles, I usually just assume they have the same problem or are too shy to say what they really think. Thankfully, I rarely encounter the passive-aggressive mumble. Shit's bunk, yo.