Lambeth Palace Library, the national library and archive of the Church of England, has a collection of medieval and renaissance manuscripts which includes MS 457, a compilation of religious matters and, on one folio, four pieces of music from c. 1200, all unique to this source. Three are complete, and two have not been previously performed or recorded to my knowledge.
This article presents a video performance of the three complete and beautiful polyphonic pieces:
Miro genere (By a wondrous birth)
Astripotens famulos (Kind ruler of the stars)
Mater dei (Mother of God)
In the video, each piece is sung in two or three voices as in the manuscript, then played polyphonically on citole or gittern. The article then explains the principles of interpreting the medieval notation.