It's already difficult to if you have a lack of self-confidence without having someone tell you constantly to speak up and stop mumbling. It may seem difficult at first, not mumbling when you speak, but learning how to stop and increasing your self-confidence is going to allow you to be more comfortable and you'll naturally speak clear.
If you mumble with just certain people, you need to look at the relationship. Often, if you're intimidated, do not have a feeling of self worth, feel like this person is constantly going to browbeat you, you're going to wind up mumbling.
So without this relationship is important, try to avoid this person and if you do run into them, try to keep your head up and answer with single syllable words.
If you find you're mumbling with just about everyone then you need to take a self-esteem course. Everyone has self-worth and you need to recognize yours. Mumbling is usually the result of either being afraid you're going to be heard, or not being confident in what you have say.
There are also a variety of different types of books you can use in order to stop mumbling. Remember, mumbling is usually a self-esteem or self-confidence issue so any books that help you develop more self-esteem and more confidence are going to help you.
The largest issue that you have is to figure out why you're mumbling in the first place. You may need a bit of professional help to dig deep into your past and figure out where your self-confidence issues lie.
But by working on your self-confidence, your self-esteem, and your self worth, you'll find that you have interesting things to say, and you'll find you naturally stop mumbling and wind up speaking clearer and with more confidence.