A study published in the Journal of Cognitive Neuroscience suggests that most readers "speak" voices in their heads while reading dialogue in books. Using MRI’s, researchers established that different parts of the brain activate as readers gloss over different parts of a book; when they read the dialogue, they create "spontaneous imagery" of the character’s voice.
According to one professor at the University of Sheffield, not hearing voices while reading is abnormal, and a lack of internal voices may be correlated with poor reading ability in individuals with dyslexia. Without a doubt, this is a promising area of dyslexia research, and with any luck, the professor’s claims will be investigated in further studies.