Hello,
I tend to have a problem with mumbling or speaking in a soft voice, unless I'm talking to those I'm comfortable with, in which case I'm much louder. Usually whenever I try to speak in that same voice with others, it comes out sounding like that really annoying, perky, girly voice that a lot of other girls tend to use (you probably know what I'm talking about).
I don't know if you mumble around everyone, but if there's a select few around whom you don't, it's a matter of becoming more comfortable with others. If you're like that around everyone, I would try practicing speaking at different vocal levels and tones, preferably when you're alone. Find a happy medium and try to stick with it when communicating with people.