Throughout my life, I have been told I mumble. My sister used to call me Mumbelina. My extremely quiet voice did not help.
I was in speech therapy for a while. Something that helped me learn to speak louder and clearer was recording my own voice. I would record my voice while talking in session and at home with others. I would then play back what was recorded. I’d hear what others heard. I had a difficult time hearing myself, even when turning the volume up high. It made me realize just how quiet and incoherent I was to others (and compared to others). Over time, it helped me a lot with the volume of my voice.
The other method I used, which helped me speak clearer, was the mirror. I would stare at the mirror and watch my mouth move as I said words and sentences. If my mouth was not expressive enough, my words were not as clear. The more I exagerated the movements of my lips and tongue, with the words, the clearer I became. I don’t like to be noticed ..so my lips would usually not move much when I talked. This helped me see what I looked like when speaking more clearly. It helped me become more accustomed to it as well.
If you can, seeing a speech/language pathologist really helps. The two methods above really improved my mumbling. Granted, my voice is still quiet and usually monotone ..but the clarity and volume has improved enough to help me interact in society. I still get the frequent “Pardon Me?” ..“Can you speak louder?” ..etc. It helps me learn ..and the person usually hears/understands me the second time.
I hope you find the answers useful.