Here's an article from their newsletter called "THE CODEX CALIXTINUS: THE MUSIC ON THE PILGRIMS’ ROAD TO SANTIAGO" that I found when Googling for some information; this topic is a very strong interest of mine. Here's an excerpt:
In liturgical terms, an analysis of the monody in the Codex Calixtinus reveals several surprising features. The typical cathedral structure of the Matins Nocturns consists of nine antiphons with their corresponding psalms and nine lessons with their responsories. The structure that appears in the Codex Calixtinus, however, is the one typically used in monasteries, consisting of twelve antiphons and twelve responsories. How can this monastic structure be reconciled with a book designed to be used in a cathedral? It may be that the Office was composed by monks for their brothers in Compostela, or simply that when the copy of the Codex was made, it was taken from a version for monastic use. The texts, taken from the Bible and from the Passio major of St James the Great, were set to music according to various procedures: some were new compositions, but others, such as the eighth responsory, Misit Herodes, are a paraphrase of both the text and the melody of the Martyrdom of St John the Baptist. The melodies of the verses to the responsories follow the usual formulae, and there is a certain similarity with the melodic structures peculiar to the region of Vézelay in some of the contrafacta, such as the antiphon of the Magnificat in the first tone, Ad sepulchrum beati Jacobi, which is simply an adaptation of the Office of St Germain in Auxerre, near Vézelay. The antiphon for the Matins Invitatory is the same as that for All Saints. The antiphon for Compline, Alleluia Jacobe sanctissime, is an imitation of the Gallican melody on Lapis revolutus (a proof of its French origin); the introit of the Mass of the Vigil, Jacobus et Johannes, is an imitation of the Christmas Mass at Dawn, Lux fulgevit; the gradual Nimis and the tract Iacobus paraphrase the types of the second and the eighth modes, respectively. The Alleluia Sanctissime apostole is an adaptation of Laetabitur iustus, while the communion Ait Ihesus is adapted from Tu est Petrus.... As we can see, many of the melodies in the Codex Calixtinus belong to the old Gregorian tradition, but are variously adapted to the texts for the Apostle James. However, the Codex does contain some surprising features, such as the tone of the psalmody accompanying the antiphon of the Invitatory of Matins for the first day, which is the first notated piece in the Codex. Only the beginning of the melody is indicated and the whole must be reconstructed by consulting books that, although originating in Aquitaine, served as reference manuals for the introduction of the Roman rite in local churches in Spain. Such is the case of an antiphonary for the Divine Office that is currently housed in the Chapter Library of Toledo Cathedral (ms. 44.2). The melody survived into the 16th century, the notation being included in an Intonarium Toletanum commissioned by Cardinal Cisneros that was printed in 1515.