On the Melisma Fabrica Mundi, and why it is sung on a rather than on another other vowel
The melismatic jubilus signifies these things, because the world’s frame is created ineffably by the Word of God. That is why the words Fabrica mundi are sung with a jubilus. It is sung on the letter “a” because it is the first sound produced by man at his birth; and because at the creation of the world the morning stars (astra) praised the Lord and all the sons of God, i.e. the angels, sang together in a great chorus. Through “a”, we imitate their first jubilus. Further, because we sing about how the Word proceeded ineffably from the Virgin as a bridegroom from his chamber (tamquam sponsus de thalamo), the letter “a” in tamquam is also sung melismatically. Because the Word is ineffably co-equal in the Father and the Holy Spirit, we sing the “a” jubilus in Gloria Patri. Since the Word is called the alpha and omega, the beginning and the end, so the melisma is sung on alpha (a) and omega (o).
CAP. VIII. – De neuma fabricae mundi. Et cur potius in a, quam in alia vocali cantetur.
Neumarum autem iubilatio est harum rerum significatio, quia ineffabiliter fabrica mundi per verbum Dei creatur. Ideo in fabrica mundi neuma iubilatur. Idcirco vero per a cantatur, quia prima vox nascentis hominis a praedicatur; et in prima creatione mundi, astra matutina Dominum laudaverunt, et omnes filii Dei, scilicet angeli, magna voce iubilaverunt: quorum primum iubilum nos per a imitamur. Et quia ineffabiliter de Virgine tanquam sponsus de thalamo processisse cantatur, ideo neuma in a tanquam modulatur: Quia vero ineffabiliter in Patre et Spiritu sancto coaequalis praedicatur, ideo neuma in Gloria Patri iubilatur. Et quia ipse a et ω scilicet principium et finis commemoratur, ideo neuma in a et ω modulatur.
On the Second and Third Nocturn
In the second nocturn we remember the shepherds’ devotion. We read how they hurried to Bethlehem and found Christ in the manger. We express this especially in the Responsories Quem vidistis and O magnum mysterium. In the third comfort and joy came into the world. Therefore in the third nocturn the Church sings the Alleluia in the antiphons more often. In the Gospels the story of the shepherds is proclaimed, through whom Christ’s birth is manifested. Women are accustomed to visit the one giving birth and bring her gifts. This is imitated in the Responsories where we greet both Christ and Holy Mary and offer, as it were, our own small gifts, something fitting for the Son, and something appropriate for the mother as well.
CAP. IX. – De secundo et tertio Nocturno.
In secundo Nocturno pastorum devotionem recolimus, quos ad Bethlehem festinasse, et Christum in praesepio invenisse legimus. Hoc maxime per responsorium, Quem vidistis, et O magnum mysterium, exprimimus. In tertio tempore gaudium et laetitia mundo advenit. Ideo in tertio Nocturno Ecclesia frequentius alleluia in antiphonis canit. Per Evangelia pastorum narratio declaratur, per quos nativitas Christi manifestatur. Solent mulieres parturientem visitare, et ei xenia deferre; has in responsoriis imitatur, dum nunc Christum, nunc sanctam Mariam salutamus, quasi congrua Filio, convenientia quoque matri munuscula offerimus.
In the Melisma Et veritate, and Why it is Sung on the Vowel e
And so we sing the words veritate with a jubilus because we praise the Word ineffably made flesh. For the first sound of a woman being born is said to be e, and the Word being born is made flesh (caro), which is a grammatically feminine word. Then the Te Deum laudamus represents for us the hour in which Christ was born, whom we praise (laudamus) with the angels. Hence the Mass comes immediately thereafter, in which we sing the Gloria in excelsis with the angels and offer the babe newly born for us as a sacrifice. After Mass we read the Book of the Generation in which, as it were, a string is tied to a fishhook and Leviathan is brought out of the human race. At this point some people sing the Te Deum laudamus, celebrate having been brought out from the jaws of Leviathan.
CAP. X. – De neuma et veritate, et cur in e.
Neumam in veritate ideo iubilamus, quia Verbum ineffabiliter carnem factum collaudamus. Prima enim vox nascentis feminae dicitur e esse, et Verbum natum est caro, quae profertur genere feminino. Deinde Te Deum laudamus repraesentat nobis horam qua natus est Christus, quem cum angelis laudamus. Ideo mox missam subiungimus, in qua Gloria in excelsis cum angelis canimus, et natum pro nobis sacrificium offerimus. Post missam Evangelium Liber generationis legitur, in quo velut linea ad hamum contexitur, quo Leviathan de humano genere extrahitur. Hic quidam Te Deum laudamus canunt, quia se de faucibus Leviathan extractos plaudunt.
 Here the jubilus is seen as an imitation of the “ineffable” song of the angels, inspired by the “ineffable” mystery of the subject of the Responsory.
 He is talking about the Responsory Descendit de cælis, which (depending on the MS) included two or three long melismas on a single syllable. In the version known to Honorius, as Durandus, it had three 3 melismas, on the (a) of fabrica, the (a) of Tamquam, and the (o) of Gloria Patri. This Responsory was not preserved in the Tridentine breviary, perhaps because it never made it into the Roman curial books. Responsories varied in the Middle Ages. Honorius’ Matins is not identical to Tridentine Roman Matins.
 Quem vidistis, pastores? dicite, annuntiate nobis, in terris quis apparuit? Natum vidimus, et choros Angelorum collaudantes Dominum.
 O magnum mysterium, et admirabile sacramentum, ut animalia viderent Dominum natum, iacentem in præsepio. Beata Virgo, cuius víscera meruerunt portare Dominum Christum.
 The 9th Responsory at Matins in Honorius’s use; the 8th in the Tridentine books: “Verbum caro factum est, et habitabit in nobis: Cuius gloriam vidimus, quasi Unigeniti a Patre, plenum gratiæ et veritate”, the last vowel is sung with a melismastic jubilus.
 The Gospel of Matthew, which begins “Liber generationis Iesu Christi…”
 The idea seems to be that Christ lowers himself from heaven into the world (the mouth of Satan) on a linea (which means both fishing line and family line) that is used to “catch” the devil. See more here and here.