S ub-vocalization is a natural process that occurs when we read. This is when we hear the sound of a word in our minds, as it is read. It is also known as silent or internal speech, since the words are not mouthed as they are being read.
Unlike reading out loud, which involves lip movement, sub-vocalization involves the movement of your larynx, also known as your voice box. Since no sound is actually made during sub-vocalization, it often goes unnoticed, even by the reader.