What did you say?   
Could you speak up please.
Whoa there partner!
Could you slow down a bit?
What did he say?  No idea he mumbled something about “come lately”.

Three main components to your voice are volume, pace and articulation.  Get these right and you will sound dynamite.
If you are naturally soft spoken, raising your voice may feel like you are yelling. To be heard you need to project, not yell at your
audience.  A simple test is to ask a friend to sit in the back of a room and speak so they can hear you. Relax and open your throat as you speak. Let the power of your voice come from your diaphragm, the area below your ribs. In short order. you will be projecting to the back row.

Nervous anxiety is the fuel which sends your mouth into overdrive.  A brief walk will dissipate the nervous energy or, more discretely, just breath in and tense all your muscles except your face (remember be discreet) and relax your muscles as you exhale.  While talking, pause for a breath. The audience will be catching up and thinking you are being thoughtful.

As children we mumbled to test our parent’s interest in the answer to their question.   As an adults mumbling just makes our audience testy.  When you speak use short words and
say them completely.

Speak up so everyone can hear you. Slow down so everyone hears your message.  Speak out clearly so everyone understands what they heard, completely not “come
lately”.