Three hours of what is to my mind the most glorious and holy form of sacred music ever composed: the Gregorian chant, other forms of plainsong, and medieval motets of the Age of Faith, when European Christendom was at its height, and maintained an essential unity which transcended the often bitter and violent bickering of rival kingdoms. The “Peace of God” in musical form!
If three hours of this celestial music is not enough for you, here is another excellent assortment, by the Tudor Consort:
Note that many of these are not in fact Gregorian chants, but polyphony. They are, nonetheless, both beautiful and holy – although not, to my mind, possessed of the simple and profound purity of true plainsong, particularly in the Gregorian mode.