From an article at the New Liturgical Movement on Vatican Singing Norms:
“The liturgy is celebrated in the Latin language, according to the Roman Rite. Gregorian chant has first place. The guest choir is expected to chant the Ordinary of Holy Mass in alternation with the Musical Chapel of the Basilica.”
“The guest choir may sing: at the Entrance procession until the moment when the celebrant reaches the altar (the Gregorian Introit is sung by the Musical Chapel of the Basilica), at the preparation of the gifts and relative offertory, at Communion, after the Gregorian antiphon has been sung, and at the end of Mass, after the Blessing. The program of music must follow the Liturgy of the day and will be agreed upon with and approved by the Choirmaster.”
And what should the choir sing? It is not complicated:
Sundays of Advent: Missa XVII Credo IV
Sundays of Christmas: Missa IX Credo IV
Sundays of Lent: Missa XVII Credo IV
Sundays of Easter: Missa I Credo III
Sundays of Ordinary Time: Missa XI Credo I
Feasts of Ordinary Time: Missa VIII Credo III
Feasts of the B.V. Mary: Missa IX Credo IV
Feasts of the Apostles: Missa IV Credo III
By the way, the norms are not so much new (Vatican document from 2006), rather newly enforced and advocated (e.g. CMAA posting the norms at the Vatican’s request).
Neat to see the Madeleine Choir School in Salt Lake City, Utah mentioned. The Cathedral of the Madeleine has beautiful art and music.