I've been re-reading "This is your Brain on Music" and while I may quibble on some moot points, it is certainly a good read for geeks & non-geeks who kind of wonder about people and music.

There are a number of interesting points raised in the book- but what I found interesting was how a scientist/former music producer who still finds time to play his instruments was troubled with how modern Western society had segregated music into two catagories - music creators and music consumers. As opposed to music created and participated in by all- something that had always been inherent in previous cultures.

So maybe the answer to the "When" question is - whenever control of music is removed from all levels of a culture and placed in the control of a select few- who may or may not actually be musically inclined.