Nowadays, the norm when reading a book alone is to read it silently to oneself.
Apparently, this practice was unusual in the ancient world.
As an undergraduate I had to read St. Augustine’s Confessions (397-400), which some claim to be the first autobiography written in the Western world. At the time, I remember being taken aback by a scene in Book VI, when Augustine comments on the strangeness of his teacher Ambrose’s practice of reading silently:
“We wondered if he read silently perhaps to protect himself in case he had a hearer interested and intent on the matter, to whom he might have to expound the text being read if it contained difficulties, or who might wish to debate some difficult question. If his time were used up in what way, he would get through fewer books than he wished. Besides, the need to preserve his voice, which used easily to become hoarse, could have been a very fair reason for silent reading. Whatever motive he had for his habit, this man had a good reason for what he did.”
According to Alberto Manguel in his book A History of Reading, this account “is the first definite instance recorded in Western literature” of someone reading silently.
There's some debate about how infrequent silent reading was in the ancient world, but it seems safe to say that reading out loud was the ancients' preferred mode.
In Space Between Words: The Origins of Silent Reading, Dr. Paul Saenger argues that reading out loud was a practical necessity in the ancient world because manuscripts didn’t have spaces between words until the end of the 7th century A.D.:
“It wasn’t literally impossible to read silently, but the notation system was so awkward that the vast majority of readers would have needed to sound out the syllables, if only in a muffled voice.”
[John 1 from the Codex Sinaiticus - the oldest copy of the Bible]
Were there any other benefits to reading out loud versus in silence? According to Nietzsche, yes. In the following passage from Beyond Good and Evil, he laments that modern readers inevitably lose some of the drama of the text when they read in silence:
“The German does not read aloud, does not read for the ear, but merely with his eyes: he has put his ears away in the drawer. In antiquity, when a man read—which he did very seldom—he read to himself... in a loud voice; it was a matter for surprise if someone read quietly, and people secretly asked themselves why he did so. In a loud voice: that is to say, with all the crescendos, inflections, variations of tone and changes of tempo in which the ancient public world took pleasure.”
For what it’s worth, I’ve found that reading out loud to my children has had numerous benefits for their own comprehension. Perhaps the growing popularity of audio books is a good thing. Have we been missing something by not hearing more literature read out loud, whether from our own voices or others?