R.C. Williams’ 1948 television paper cites an article by Richard Hughes from the  year before: ‘The Second Revolution: Literature and Radio‘.¹ Hughes raises a number of issues that I.A. Richards and Eric Havelock were discussing around the same time and therefore shows how these were in the air prior to McLuhan’s detailed treatment of them beginning in Explorations, but especially in The Gutenberg Galaxy, some 15 years later:

• Before the printing-press was invented, the writer reached the majority of his public not through their eyes but through their ears. Poetry was sung or recited; prose books, too, were recited or read aloud. Not only primitive communal literatures such as the Homeric cycle, the Sagas, and the Mabinogion; at a much later stage than that, long after the poet took to composing with stylus or pen in hand instead of drum or lyre, he still wrote not to be read but to be heard. (34)
• The lovely illuminated manuscripts of the medieval monasteries were meant to delight the eye, but to be looked at rather than to be read— at least, not read in the sense of passing round the monastery from hand to hand. Their text was read aloud in the refectories, or sung in the Churches, rather than pored over in the cells. The language of King James’s Bible (as well as the English Prayer Book) was so intended. For the effect of the printing press on literary on literary style was profound but it was not sudden. It was a slow development, culminating only in our own century.  (34-35)
• Gradually, in the intervening time, poetry acquired a subtler intricacy as the poet found he need no longer rely on the immediate aural impact of word added one by one to measured word. (…) By the same token, such poetry had to be banished from the stage. In earlier days poetry had seemed the natural mode for the stage, since the poetic was par excellence the mode of utterance aloud, In Caxton’s own day, John Skelton described himself as “Poet” and “Orator” almost interchangeably. (34-35)
• Prose likewise developed a greater elaboration of structure, rolling out interminable periods, gorgeous and majestic to the eye, which on the tongue would have taxed the lungs of Aeolus, In short, there grew a split in style between the art of the spoken and of the read word: between oratory, an art which has extension only in time, and literature, which has extension in space coupled with a time-dimension which the reader can manipulate at will… (35)
• Reading aloud died hard, barely a generation ago. (…) Thus the last echoes of heard literature had died away, but had only just died away, when a second revolutionary invention, wireless broadcasting, set the pendulum swinging again in the opposite direction. The Voice had come back. (35)
• It may be argued, not implausibly, that radio will be the only literature of the future; that the present age of universal literacy is only a passing phase; that in a generation or two reading and writing will be dead like Greek and Latin, and dead for the same reason — that they will no longer be necessary for daily life. (42)