On this day in 1170, the Archbishop of Canterbury Thomas Becket was murdered at the altar by knights of King Henry II. It was the culmination of a long running power struggle between these two once fast friends become adversaries.
The King had exiled Becket in 1164, and Becket had waged a PR war against the King the entire time. Henry lost that, and came under increasing pressure to allow Becket to return. Only a few months after that return, Henry's temper got the better of him, and the story played to its bloody end.
This song is part of Becket's PR campaign. The title translates to "the sound of weeping is heard in Rama", it was a veiled allusion to his exile. This places it between 1154 and 1170, an unusually precise dating for such an early piece of music.