With the implementation of the new translation of the Roman Missal which we began using in Advent, many have questions about why we sing what we sing at Mass. There are both conceptual and practical reasons for choosing what parts of the Mass to sing. Let's just deal with the conceptual reasons this week, and leave the practical considerations for next week.
The Roman Rite, the Mass we celebrate, allows for the possibility of literally everything being sung (the ideal, and when we say everything, we mean everything, even the readings!), as well as for the possibility that nothing is sung, but instead spoken, and every degree in between. In determining what to sing and what not to sing, the Church basically takes into account two things. First, since singing naturally enhances the solemnity of a particular moment, we consider the level of importance in relation to other moments. And second, how much the singing is married to the ritual action. In general, these parts should be sung before any other: 1) The dialogues between the priest and the people (ie, "The Lord be with you," and, "And with your spirit," 2) the Kyrie and Gloria, when appropriate, 3) The Alleluia and dialogues before and after the Gospel, 4) the Sanctus, 5) the Mystery of Faith, 6) the Final Doxology and Amen, 7) the Lord's Prayer, and 8) the Agnus Dei. Other things that could be added include the Responsorial Psalm, the Universal Prayer, the Preface and Eucharistic Prayer, and the Profession of Faith (ie, the Creed). The last sung parts to be considered are the Entrance, Offertory, and Communion processions.
Looking at the degrees above, it's clear the Church places high importance on singing parts where the singing is the primary liturgical action (for instance, the Gloria, the Sanctus, the Lord's Prayer), as well as when singing is closely tied to the primary action, such singing the Alleluia during the procession of the Book of the Gospels. Next week we'll examine choosing the sung parts from a practical perspective.