Barbershop Quartets and other singing groups that hold chords, are aware of an extra voice that appears when the voices are all on pitch. It sounds like is a high note that is a common harmonic to all the notes that are being sung--

It is, more or less an illusion--every note has a series of harmonic overtones that sounds above it at the same time--when the voices in a group are on pitch, and they sing a chord, some of these harmonic overtones blend, as well, and they create the feeling of an extra voice.

On a dulcimer, you can sound harmonics on each of the strings by damping the strings at the exact midpoint of the neck, and striking the strings. These notes are all sounding when the strings sound, but the fundamental note that the string plays overpowers them--