New York's Ensemble for Early Music - Daniel and the Lions - Ludus Danielis (2004) [Hi-Res]

The words medieval opera were used jokingly in reference to medieval mysteries, crude performances staged by various monasteries and merchant guilds both for entertainment and to teach the content of some biblical stories.
Since the Bible had not yet been translated into any local languages, and the services were conducted mainly in Latin, the mystery was an immediate method by which an ordinary person could get an idea of the contents of the Bible.
Although music was part of the usual mix of medieval mystery, some were very passionate about music, and Ludus Danielis (Daniel's Play), copied around 1230 into the British Library manuscript Egerton 2615, but dating from the middle of the XII century, is one of the earliest and most detailed sources of this kind.
Medieval mysteries were not operas; they were aimed at the townspeople, not the nobility, and the usual division of numbers into arias, choruses and recitatives, common to opera, did not exist in such works.
Given some signs of instrumentation and stage acting, Ludus Danielis still sounds good in the twenty-first century and has been revived many times, first by the famous Noah Greenberg in the late 50s.