All of these remarks suggest that the instrumental and vocal music were identifiably distinct and probably did not occur simultaneously. Polyphony, which simultaneously combines several lines of melody in parts, without any line subordinate to the others, did not begin to develop until near the end of the Middle Ages. In The Silmarillion Iluvator utilizes polyphony when he teaches the Holy Ones to sing in parts and then adds another part of his own, the third theme for humans. In all likelihood, the singing of the Holy Ones represents the most advanced type of music in Tolkien's worlds, a type that had not yet made a general appearance in Middle-earth in the Third Age. Given that polyphony was at best rare in Middle-earth at the time of LOR , an authentic reconstruction of a song should be a single melody line without instrumental accompaniment.
Percussion is also a problem.
Because the notation for the songs in Gregorian chant from the Middle Ages does not indicate the lengths of notes, it is not known exactly how any particular song was sung. It is not unlikely that each song came out differently each time it was sung, just as a passage from a book comes out differently each time it is read aloud. Most likely, the singer deliberately tried various approaches to the presentation of particular lines each time he or she sang the song. The object would be to display the natural rhythm of the line in contrast to the rhythm of the other lines. In measured music, each line is rhythmically identical.