Augustine on Ambrose, Confessions 6.3
“But when he [St. Ambrose] was reading, his eyes were lead along the pages and his heart was revealing their sense, but his voice and tongue maintained silence. Often, when we were in his presence (for he prohibited no one from coming in, nor was it his habit that an entrant should be announced to him), we would see him reading silently – never otherwise. After sitting in unbroken silence (who, indeed,would burden a man so intent on something?), we would leave him. We conjectured that when he had seized upon that small interval of time for rejuvenating his mind, taking a break from the tumult of other people’s affairs, he did not want to be summoned away to something else, so he took care lest some eavesdropper get caught up in it would hear something which the author of the book being read had expressed unclearly, thus making it necessary for Ambrose to explain and discuss various difficult questions. Then, by spending his free minutes on such a task, he would get through fewer books than he wished, even though the cause of preserving his voice, which was easily wrecked, had been a more just reason for reading silently.”
sed cum legebat, oculi ducebantur per paginas et cor intellectum rimabatur, vox autem et lingua quiescebant. saepe cum adessemus (non enim vetabatur quisquam ingredi aut ei venientem nuntiari mos erat), sic eum legentem vidimus tacite et aliter numquam, sedentesque in diuturno silentio (quis enim tam intento esse oneri auderet?) discedebamus et coniectabamus eum parvo ipso tempore quod reparandae menti suae nanciscebatur, feriatum ab strepitu causarum alienarum, nolle in aliud avocari et cavere fortasse ne, auditore suspenso et intento, si qua obscurius posuisset ille quem legeret, etiam exponere esset necesse aut de aliquibus difficilioribus dissertare quaestionibus, atque huic operi temporibus impensis minus quam vellet voluminum evolveret, quamquam et causa servandae vocis, quae illi facillime obtundebatur, poterat esse iustior tacite legendi. quolibet tamen animo id ageret, bono utique ille vir agebat.
Reading aloud is often viewed with some derision in the modern world, but it was the default mode of reading in antiquity. Augustine (4th-5th centuries BCE) is a comparatively late author, yet found Ambrose’s habit of silent reading sufficiently remarkable to record here.
I confess that I usually read silently, yet I find that it is – in contradistinction to the suggestion here – the surest way to guarantee that someone will assume that you are at your leisure and effectively begging to be interrupted.