What is the neurobiological basis of the "inner voice" used for thought or reading?
Preliminary answer I intend to improve later. Stumbled upon this in the context of description of how people read:

According to this paper on readin (Sousa, 2005), novice readers internally verbalize written words of English and German using "an area of the brain just above and behind Wernicke's area". This area then communicates with the Broca's area and the frontal lobe.

Found this additional article which mentions that people use the same area of the brain when reading silently as when listening actively.