I really don't want to get involved with the Jim Carroll show. I'm more interested with the word, and it is fascinating stuff.
Following from Jack Campin's post I get that the subject of the link is not at all straightforward. So I tried looking at Chaucer's words;
The nearest is 'mediatour' which seems to be an obsolete form of, you guessed it, mediator. Am I right to assume that in the musical context it means 'a person who explains'? Is it that simple?