We sing because there are truths that we cannot express any other way — the sublime beauty of God, the joy of our experience of His love, our deep gratitude for the Eucharist. We can come to know God numerous ways, including through the transcendentals of truth, goodness, and beauty; when beautiful chants or choral music are sung at Mass, they lift us out of ourselves and point to the Author of all beauty. Liturgical music is meant for the glory of God and the edification & sanctification of the faithful.
The Church has published many documents containing guidelines for sacred music, most notably 1903's Tra le sollecitudini, which called for the return to Gregorian chant as the normative music of the church, and 1963's Sacrosanctum concilium, the liturgical constitution of the Second Vatican Council, which repeated this call.
The earlier document states, "the more closely a composition for church approaches in its movement, inspiration, and savor the Gregorian form, the more sacred and liturgical it becomes; and the more out of harmony it is with that supreme model, the less worthy it is of the temple." The Church understands Gregorian chant to be the template by which the appropriateness of all other liturgical music must be evaluated. Vatican II's later document echoes this requirement, and takes it a step further, calling for thorough musical and liturgical training of children, clerics, and choirs; it also grants the liturgical use of "other kinds of sacred music, especially polyphony." The use of polyphony is a natural extension of Gregorian chant, since that repertory is based on the interaction of chanted or chant-like melodies woven together into a wondrous and rich sonic tapestry.
What about Latin? While Vatican II permitted the use of the vernacular language in liturgy, people may be surprised by its statement that "the use of the Latin language is to be preserved in the Latin rites." The liturgical constitution further exhorted that "steps should be taken so that the faithful may also be able to say or to sing together in Latin those parts of the Ordinary of the Mass which pertain to them." The Council expected the bulk of liturgical worship, including much of its music, to remain in the Church's traditional tongue, Latin.