Today is the feast of Saint Michael the Archangel. What we actually celebrate is the dedication of his church on Monte Gargano. Closely related to this feast is the Apparition of Saint Michael, which also happened on Monte Gargano, formerly celebrated on 8 May (prior to the removal of duplicate feasts in the General Roman Calendar), the prelude to the dedication of the church which we celebrate today.
We associate Saint Michael, whose name means Who is like God? (מי כאל), with the defeat of the Devil. This is invoked in the prayer which we usually pray at the end of every Low Mass, after the Salve, Regina and the versicles. Lately, in parts of the Philippines, this prayer experienced a renaissance, and is now said after each Mass. Even in Sung Masses, this prayer is now recited immediately after the recessional hymn.
That said, here is a 20th-century plainchant setting of the prayer from Cantus varii. It is in fourth mode, the Church’s quintessential bittersweet mode (click the thumbnail on the right to open the file).
Here is also a later plainchant setting of the same prayer from Heiligenkreuz, cheekily touted as coming from a 21st-century manuscript thither. This one is in the first mode (click the thumbnail on the left to go directly to the file in Scribd), and is intended to be chanted as part of the Leonine prayers at Low Mass. Here is a recording.
Sancte Michaël Archangele, defende nos in prœlio !