O Virgin of Virgins, how shall this be? For neither before thee was any like thee, nor shall there be after. Daughters of Jerusalem, why marvel ye at me? That which ye behold is a divine mystery.
There is no available sound file for this one; the only people who sing it are Anglican and Norbertine monastics - and neither has recorded it, apparently. By now, though, you know the tune, no doubt - so have a go at it yourself; Gregorian Chant is meant to be sung and not merely listened to!
More about "O Virgo Virginum" here and here.
A blessed Feast of the Incarnation to all.