I found this on Youtude, it is Ambrosian Chant, of which I know little. If you share my ignorance you might find this by the clip's poster micrologus2 useful:
Ambrosian chant (also known as Milanese chant) is the liturgical plainchant repertory of the Ambrosian rite of the Roman Catholic Church, related to but distinct from Gregorian chant. It is primarily associated with the Archdiocese of Milan, and named after St. Ambrose much as Gregorian chant is named after Gregory the Great. It is the only surviving plainchant tradition besides the Gregorian to maintain the official sanction of the Roman Catholic Church.And here is something even more esoteric: A Mozarabic Gloria.
Ambrosian chant is largely defined by its role in the liturgy of the Ambrosian rite, which is more closely related to the northern "Gallic" liturgies such as the Gallican rite and the Mozarabic rite than the Roman rite. Musically, however, Ambrosian chant is closely related to the Gregorian and Old Roman chant traditions. Many chants are common to all three, with musical variation.
Like all plainchant, Ambrosian chant is monophonic and a cappella. In accordance with Roman Catholic tradition, it is primarily intended to be sung by males, and many Ambrosian chants specify who is to sing them, using phrases such as cum Pueris (by a boys' choir) and a Subdiaconis (by the subdeacons).
Stylistically, the Ambrosian chant repertoire is not generally as musically uniform as the Gregorian. Ambrosian chants are more varied in length, ambitus, and structure. Even within individual categories of chant, Ambrosian chants vary from short and formulaic to prolix and melismatic, and may be freely composed or show significant internal melodic structure. Its most distinctive feature compared with other plainchant repertories is a significantly higher amount of stepwise motion, which gives Ambrosian melodies a smoother, almost undulating feel. In manuscripts with musical notation, the neume called the climacus dominates, contributing to the stepwise motion. More ornamental neumes such as the quilisma are nearly absent from the notated scores, although it is unclear whether this reflects actual performance practice, or is simply a consequence of the relatively late musical transcription.
The Gregorian system of modes does not apply to Ambrosian chant. Although there are no b-flats indicated in the musical notation, it seems likely that they were understood, based on Guido d'Arezzo's description of the "more perdulcis Ambrosii."
Nearly all of the texts used in Ambrosian chant are biblical prose, not metrical poetry, despite Ambrose having introduced Eastern hymnody to the West. Ambrosian chant serves two main functions in the Ambrosian liturgy: to provide music for the chanting of the Psalms in the monastic Offices, and to cover various actions in the celebration of the Mass.