It is the Second Sunday of Advent and the Introit is Populus Sion.
PEOPLE of Sion, behold the Lord shall come to save the nations: and the Lord shall make the glory of His voice to be heard, in the joy of your heart. Ps. Give ear, O Thou that rulest Israel: Thou that leadest Joseph like a sheep. ℣. Glory be to the Father.
But you won't here it because Cardinal Thomas Collins has decreed that there will be:
Please enjoy this setting of the Introit by Heinrich Isaac, 1450-1517.