Is Sung Matins a form of mass?
No, but Matins does form part of the Divine Office
Matins is the first official set of prayers that all religious and all ordained members (priests and deacons) of the Church must recite each day. Matins is followed by Lauds, Prime, Terce, Sext, None, Vespers and Compline. Some communities prefer to recite or sing (Chant) their Matins in the evening. In this case it is called Vigils instead of Matins. Many priests today are exempt from praying the smaller hours or Terce, Sext and None. Prime in our modern times has almost vanished all together.
The custom of reciting prayers at certain hours of the day or night goes back to the Jews, from whom Christians have borrowed it. In the Psalms we find expressions like: "I will meditate on thee in the morning"; "I rose at midnight to give praise to thee"; "Evening and morning, and at noon I will speak and declare: and he shall hear my voice"; "Seven times a day I have given praise to thee"; etc. (Cf. "Jewish Encyclopedia", X, 164-171, s.v. "Prayer"). The Apostles observed the Jewish custom of praying at midnight, terce, sext, none (Acts 10:3, 9; 16:25; etc.). The Christian prayer of that time consisted of almost the same elements as the Jewish: recital or chanting of psalms, reading of the Old Testament, to which was soon added reading of the Gospels, Acts, and Epistles, and at times canticles composed or improvised by the assistants. "Gloria in excelsis" and the "Te decet laus" are apparently vestiges of these primitive inspirations. At present the elements composing the Divine Office seem more numerous, but they are derived, by gradual changes, from the primitive elements. As appears from the texts of Acts cited above, the first Christians preserved the custom of going to the Temple at the hour of prayer. But they had also their reunions or synaxes in private houses for the celebration of the Eucharist and for sermons and exhortations. But the Eucharistic synaxis soon entailed other prayers; the custom of going to the Temple disappeared; and the abuses of the Judaizing party forced the Christians to separate more distinctly from the Jews and their practices and worship. Thenceforth the Christian liturgy rarely borrowed from Judaism.
The development of the Divine Office was probably in the following manner: The celebration of the Eucharist was preceded by the recital of the psalms and the reading of the Old and New Testaments. This was called the Mass of the Catechumens, which has been preserved almost in its original form. Probably this part of the Mass was the first form of the Divine Office, and, in the beginning, the vigils and the Eucharistic Synaxis were one. When the Eucharistic service was not celebrated, the prayer was limited to the recital or chanting of the psalms and the reading of the Scriptures. The vigils thus separated from the Mass became an independent office. During the first period the only office celebrated in public was the Eucharistic Synaxis with vigils preceding it, but forming with it one whole. In this hypothesis the Mass of the Catechumens would be the original kernel of the whole Divine Office. The Eucharistic Synaxis beginning at eventide did not terminate till dawn. The vigils, independently of the Eucharistic service, were divided naturally into three parts; the beginning of the vigils, or the evening Office; the vigils properly so called; and the end of the vigils or the matutinal Office. For when the vigils were as yet the only Office and were celebrated but rarely, they were continued during the greater part of the night. Thus the Office which we have called the Office of evening or Vespers, that of midnight, and that of the morning, called Matins first and then Lauds, were originally but one Office. If this hypothesis be rejected, it must be admitted that at first there was only one public office, Vigils. The service of eventide, Vespers, and that of the morning, Matins or Lauds, were gradually separated from it. During the day, Terce, Sext, and None, customary hours of private prayers both with the Jews and the early Christians, became later ecclesiastical Hours, just like Vespers or Lauds. Compline appears as a repetition of Vespers, first in the fourth century (see Compline). Prime is the only hour the precise origin and date of which are known--at the end of the fourth century (see Prime).
At all events, during the course of the fifth century, the Office was composed, as today, of a nocturnal Office, viz. Vigils - afterwards Matins - and the seven Offices of the day, Lauds, Prime, Terce, Sext, None, Vespers, and Compline. In the "Apostolic Constitutions" we read: "Precationes facite mane, hora tertia, sexta, nona, et vespere atque galli cantu" (VIII, iv). Such were the hours as they then existed. There are omitted only Prime and Complin, which originated not earlier than the end of the fourth century, and the use of which spread only gradually. The elements of which these hours are composed were at first few in number, identical with those of the Mass of the Catechumens, psalms recited or chanted uninterruptedly (tract) or by two choirs (antiphons) or by a cantor alternating with the choir (responses and versicles); lessons (readings from the Old and New Testaments, the origin of the capitula), and prayers (see Breviary). - Divine Office