Medieval Musical Literacy

Many years ago I listened to a lecture from an eminent scholar in which he equated the advancement of the world's knowledge during the course of the middle ages with the volume of manuscript material in the libraries of Europe. Apart from a drastic Eurocentric cultural insensitivity, it expressed an entirely modern concept of knowledge; that it is necessarily written down. Vast amounts of knowledge, especially practical knowledge, were simply not recorded in writing in the medieval period.
Recently I encountered an equally strange academic furphy, in which a musicologist expressed the idea that music in the 10th century was extremely primitive, because the only manuscripts which recorded musical notation displayed only monophonic plainchant. Now this, of course, was not because it was the only music around, but because the monks and clerics of that era were the only people who wrote music down, and what they wrote was the monophonic plainchant used in their offices and rituals. They were of an ascetic turn of mind, in music as in other aspects of life. We have absolutely no idea what wild, complex and exotic music was being produced by the illiterate minstrels who were entertaining the lay population.
We do know that they had a range of instruments in the medieval era, as these have been depicted prolifically in manuscripts, paintings and carvings. They must have played something on them, even though we have no instrumental musical scores. Like so many other aspects of life, they remembered a lot. The use of musical notation became more common, and orderly, in a similar timeframe to the use of lay literacy in reading and writing. But while we may know some old tunes from written sources, we don't know anything about their musical arrangements. In music, as in other areas, it doesn't do to equate written sources with knowledge.
With the increasing enthusiasm for pub sessions among folk muso types and music festival goers, we may be once again going back to the middle ages with more reliance on our ears and memories, and less on little black marks on pages.