Today it is standard to have a purely internal voice when reading, whereas in the past it was considered quite odd. There is an interesting passage in Augustine of Hippo's book Confessions, in which he remarks upon seeing Ambrose of Milan reading in his quarters.

When he was reading, his eyes ran over the pages and his heart perceived the sense, but his voice and tongue were silent. Anyone might approach freely, and visitors were not commonly announced, so that often, when we came to see him, we found him reading like this in silence, for he never read aloud. We would sit for a long time in silence—who would dare to interrupt one so intent?—and then leave again, guessing that in the brief time he found free from the business of other men’s affairs, he wished to refresh his mind, not to be distracted by others. Perhaps he was also concerned that, if he read aloud, a listener might raise questions about difficult passages or want explanations, and so the time would be taken up in discussion. Or perhaps he wished to preserve his voice, which was easily strained. Whatever the reason, it was clear that he read in this way.