I remember in my medieval philosophy class my professor once mentioned that silent reading wasn’t always the norm and that rather people would read outlouad typically. One of the most well known examples of this is in Augustine’s Confessions where he remarks that he was astonished at how Ambrose of Milan would read books silently and without moving his lips or mouth at all. It wasn’t that people didnt have the ability to read silently like people would be aware if a situation called for them to read the contents quietly such as a politician or general recieving a letter with sensitive information. The point rather is that in those cases the person would intentionally will themselves to read quietly and if you were going to sit down and read a book or scroll the default was that you would read it out loud so someone that read silently as their norm would be seen as ‘odd’. And by ‘odd’ I dont mean that people thought they were a freak or stupid just that it was a strange quirk.
As a matter of fact reading silently might’ve been seen as a sign that someone was an incredibly heavy reader such as with Ambrose. The reason being that one of the reasons vocalized reading was the norm had to do with how people wrote texts like it was pretty common for there to be irregularities with the script or especially that it was highly common for writings tonothavespacesinbetweenwordssoreadingoutloudfeltlikeamorenaturalasawaytomakeouttheindividualwords. An incredibly ‘veteran’ reader like Ambrose (Augustine also mentions Ambrose could read quickly) might develop mental shortcuts letting them more easily pick part the individual words that had been squished together and in doing so simply gradually drop the habit of reading out loud because it wasnt an aid to them any longer. In Latin Europe this changed when Irish monks in the late 7th century developed the practice of writing with more uniform letters as well as separating different words by leaving blank spaces between them. This slowly spread to the rest of Latin Europe (at least with the monastaries) until it finally became the norm in the 12th century, just in time for the explosive importation of scientific and philosophical literature from the Arabic world which was nice.
Anyway back to before that happened, something my professor said that was pretty interesting was that that if you stepped into a medieval library before this change ocurred is that youd be struct by how much talking was seeming to go on the monks read to themselves. That combined with the fact that the expensive nature of books often meant might could be attached to chains meant that a medieval library would be have a continuous din of murmuring and rattling metal which is interesting imagery for a library.
i edited this post bc i wrote it on my phone at first and some of the wording was awkward + added a little more info
also if youre interested in this then look up the historian paul saenger he’s written a bunch about this especially the relevance of word spacing