Just because one may find only a melody line written down does not mean there were not other things going on.

Many early music groups these days are concerned with the matter of authenticity, and want to perform the music with the same sound and in the same style in which it was originally performed. One aspect of this is using actual instruments of the period or the most accurate modern replicas they can find. Beyond this, they have the music itself to work from. And wherever possible, facsimiles of the music manuscripts in the composer's own hand, sometimes (although rarely) complete with dynamic markings and marginal notes.

However—much of this is left up to the individual musicians. Many works were written with melody line only, sometimes accompanied by a number or a couple of numbers under the staff and beneath each melody note. This is what is called a "figured bass," and suggests chords or chord inversions to the musicians who are not carrying the melody line, but playing in "parts."

Oftentimes the instruments themselves were not specified. You could use whatever instruments you had at hand, and pieces of early music may not always be played twice in a row with the same instruments or the same harmony lines. There was an improvisational quality to this kind of playing. Different in style, but not in approach to jazz, in which one starts with a melody line, then the musicians improvise around it.

A group of friends getting together to play—whatever instruments they happened to have, be it a "case of viols," and/or a lute or two, and/or a flute or "case of recorders," and/or a virginal (a small, portable harpsichord-like keyboard instrument that could be set on a table top). This sort of playing was an early form of what later became known as "chamber music."

This is not speculation on my part, folks. You can look this up in any good text on the history of music.

I cannot imagine that "the folk" were so isolated that occasional members of the class never heard music of this kind. Or that they were so unimaginative that it never occurred to them to try part-singing. After all, in churches and monasteries sometime in the 1100s, a bass line was added to plainchant. This was partly to accommodate singers with lower voices (basses and some baritones) who had trouble reaching the melody notes sung by the higher voices (light baritones, tenors).

Please don't try to tell me that peasants and serfs never heard any of this.

Don Firth