In The Concept of Mind, Gilbert Ryle says "It was not until the Middle Ages that people learned to read without reading aloud" (pg. 27), but he provides no citation for this claim.
Is this really true? When Plato and all the Ancient Greeks read anything, they read aloud?
Much more can be written, and personally I'd like to see a synthesis of current thinking. You might like to see some existing discussions. I found these, and if read in this order, Saenger is explained before answers to him:
"Was it really considered rare and impressive to read silently in the 4th century AD?" . /u/capehart_karsh briefly summarized Paul Saenger, The Space Between Words: the Origin of Silent Reading, 1997, which applies to that claim. /u/wedgeomatic addresses a common anecdote of Saint Ambrose, with a child discussion about common use of reading aloud.
"Is It True That People Did Not read Silently (In Their Heads) Until The Dark Ages?" . /u/Astrogator addressed scriptura continua in Latin, and husky54 addressed reading in the Near East.
"Shift from reading out loud to reading silently" . [deleted] brings in details on Gavrilov and Burnyeat.
"When did silent reading become common?" . Whoosier cites William A. Johnson, a writer who tried to address both Saenger and Gavrilov & Burnyeat.
This is not to discourage discussion. Further questions, data, and debate are welcome.