Broad scale literacy among the general population didn't really come about until well after the invention of the printing press. While not necessarily inconsistent with the hypothesis, I think it is somewhat of a cheat to refer to Bible passages that seem to indicate that at some point there was a prejudice against people who hear voices in their heads, meanwhile omitting all of the prejudices in their favour, like all of the Jewish prophets that hear the right voice of the true God. If it had been a normal thing in the distant past, people who had that "gift" would not be viewed as special, as enlightened or as prophets or as a threat. The fact that we find that they were to me speaks to this having been a rather rare thing at the time and it is not inconceivable to me that while at the beginnings of writing it was revered and considered a blessing, that as time showed more examples of unpredictable or dangerous behaviour, or false prophesies, or attributions of the voices to gods people around didn't acknowledge quite enough, people came to shun the 'gifted', or at least some of them, and view them as cursed or sick or possessed instead. Neither perception betrays it having been a normal thing.
As to my earlier remark about shocking numbers of people still communicating with gods or spirits, I must admit I did not look up the exact numbers as I didn't consider it to be a controversial proposition. I am thus willing to retract it pending further information, and reduce it to my personal anecdotal account for now. I have heard an unspecified number of mostly religious people reporting to speak to their respective god or gods regularly and frequently and receiving answers, too. Granted, few would admit to having had an auditory experience of their gods' response when pressed, more often they report that their prayers are answered in feeling they couldn't put into words or through events in their lives. Nobody admits that the conversation is wholly one-sided at first, and almost nobody admits it later on, when pressed.
Better to leave questions unanswered than answers unquestioned
