The author didn't say he couldn't speak in his mind but that he couldn't hear the sound of their voice in their head.
You can vocalize thoughts and be conscious in a linear train of thought way of what you are saying but not hear them.
This difference is highlighted when you try to sing mentally or play a musical instrument in your head. It's more about the feeling associated in your vocal cords than the feeling associated with your eardrums.
Imagine playing the violin of a melody you have practiced tons of time, you feel
every little intonation in your hands, you can feel the emotions you try to give to the music, but hear absolutely nothing. It's like if you where playing the violin with some earplugs.
You can vocalize thoughts and be conscious in a linear train of thought way of what you are saying but not hear them.
This difference is highlighted when you try to sing mentally or play a musical instrument in your head. It's more about the feeling associated in your vocal cords than the feeling associated with your eardrums.
Imagine playing the violin of a melody you have practiced tons of time, you feel every little intonation in your hands, you can feel the emotions you try to give to the music, but hear absolutely nothing. It's like if you where playing the violin with some earplugs.