You've got to open your mouth when you speak.
Yeah, I know, everybody opens their mouth when they talk, right? But haven't you seen how some people mumble with their mouth barely open? Almost as if they have their hand over their lips.
If you've got something to say, open your mouth and speak up. Don't cover the sound from escaping.
Instead, open your mouth and talk from your diaphragm. What's a diaphragm, you might ask. Well, here's a good trick. Put your hand flat on your belly. Now, say “Ho-ho-ho” loud. Your hand should rise and fall with the strongly said words. Try it again, say the words softly, then strong, pulsing your words out. This is a good exercise to see if you're talking with strength or not. Now, open your mouth and speak anything you want, using your diaphragm muscles. Or belt out “Ho-ho-ho” and see who's listening. That's always fun.
Remember, speak loud, strong, and confident!