The speech impediment got progressively worse, until Jones refused to talk to anyone outside of his immediate family. Then it got even worse until, at 8 years old, he simply stopped talking altogether. From the time he was 8 until he was 15, Jones was completely mute.
In actuality, it turns out his voice was just hibernating ...
... so that it could one day emerge from its chrysalis as goddamn Darth Vader.
Though he didn't so much as utter a word for nigh on eight years, Jones was still fascinated by words of the written variety. His teachers tested him solely through written exams, and Jones was a talented poet. A high school English teacher spotted that talent, and leveraged it into a possible cure for Jones' self-inflicted muteness: "If you like the words, you've got to be able to say them," he said, encouraging Jones to read his poetry in front of the class.
James Earl Jones
Because dunking on fools is way more satisfying when you can trash talk them afterward.
And when he did, the voice that came out was completely stutter-free. It had also changed and grown during those years of going completely unused -- what boomed out was the voice that we all know today: a rumbling bass that circled the classroom like a well-tuned Harley and presumably kick-started puberty in any students present who hadn't hit it yet.