Some of the very earliest examples of pieces of music include Plainchant. Also known as Gregorian Chant or Plainsong, Plainchant was the mainstay of music in the early church. Songs have Latin text and are sung both unaccompanied and in unison. Plainchant has no regular metre (set beats in a bar) the chant follows the natural rhythm of the text with many of the individual syllables having their own musical note. Originally Plainchant would have been sung by monks as part of worship.
Listen to 'O Virgo Splendens' while watching the original manuscript.
Hildegard von Bingen: O pastor animarum.
Codex Franus - Schola Gregoriana Pragensis