Without a doubt, the voice is an integral part of the Divine Liturgy. Whether it be a response, a prayer, or singing, the voice is a part of the Mass. The human voice becomes, especially in the Holy Mass, an instrument through the ministry of the Mystical Body to participate in that beautiful and sacred “exchange of man’s (really Christ’s) homage and Gods life” . It only seems necessary, then, that the voice partakes in the Mass in the most proper way–the most beautiful and majestic way fit for honoring the King of kings. We can come to know what is best through the Spirit that works through the Church. It is this post’s purpose, thence, to present that the most authentic praise and song fit for the Divine Liturgy is founded in the form of Gregorian Chant–as has been taught and continues to be affirmed by Mother Church.
Pope Pius X writes, in his Motu Propio Tra Le Sollecitudini promulgated in 1903, that
[Gregorian Chant is] the Chant proper to the Roman Church, the only chant she has inherited from the ancient fathers, which she has jealously guarded for centuries in her liturgical codices, which she directly proposes to the faithful as her own, which she prescribes exclusively for some parts of the liturgy, and which the most recent studies have so happily restored to their integrity and purity.
On these grounds Gregorian Chant has always been regarded as the supreme model for sacred music, so that it is fully legitimate to lay down the following rule: the more closely a composition for church approaches in its movement, inspiration and savour 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. 
Later, in 1955, Pope Pius XII wrote an encyclical, Musicae Sacrae, which delivered a similar ethos. For-instance, the Bishop explains that sacred music must be “holy”. He then declares that “Gregorian chant…is gloriously outstanding for this holiness”. 
At this point, it is most likely that one will point out that these quotes thus far are “pre” Vatican II. However, I think that kind of language is not very Catholic, to be completely honest. As a Catholic, we believe that the Church is one. There are never “pre” and “post” churches. Certainly, every council represents a visible moment in Church history that signifies some sort of renewal, message, etc. However, the Church is the Church, and it is imperative, when we consider matters of the Church–including theological, liturgical, and historical–that we dialogue using a hermeneutic of continuity .
Furthermore, I think that a reading of some of the documents from Vatican II will prove this continuity of Gregorian chant as the preferred and suggested musical choice. In the Council’s Constitution on the Sacred Liturgy, the text states: “The Church acknowledges Gregorian chant as specially suited to the Roman Liturgy: therefore, other things being equal, it should be given pride of place in liturgical services” . Vatican II agrees with past teaching and continues to declare the official music of the Roman Church’s Liturgy: Gregorian chant.
At this point, I simply want to offer a few more quotes that uphold Vatican II’s teaching:
From Musicam Sacram: “According to the Constitution on the Liturgy, “the use of the Latin language, with due respect to particular law, is to be preserved in the Latin rites” . Note that this is referencing a Vatican II document. Contrary to popular belief, Vatican II was a Latin Church council–and Latin in the full meaning of the word. Not only were all of its documents promulgated in Latin, but it also upholds the use of Latin the Roman Church.
In a Letter to the Bishops on the Minimum Repertoire of Plainchant, the Sacred Congregation for Divine Worship suggests that “all the faithful should know at least some Latin Gregorian chants, such as, for example, the ‘Gloria’, the ‘Credo’, the ‘Sanctus’, and the ‘Agnus Dei'” .
From the General Instruction on the Roman Missal (2002): “All other things being equal, Gregorian chant holds pride of place because it is proper to the Roman Liturgy. Other types of sacred music, in particular polyphony, are in no way excluded, provided that they correspond to the spirit of the liturgical action and that they foster the participation of all the faithful” .
I might also add that a simple look to the Church in Rome, at the Bishop of Rome’s writings and celebrations: it is clear that His Holiness Pope Benedict XVI respects these teachings, specifically, the primacy of chant and the use of Latin.
Might I end with a great quote from John Miller, C.S.C. Exploring the nature and definition of the Liturgy, he concludes:
[T]he liturgy is the public (in the sense of being done by and for the whole Mystical Body) worship of the Mystical Body in the entirety of its Head and members, worship which is Christ’s prayer and action, effecting a holy exchange of God’s life and human homage…worship which is concretized in the Mass, the sacraments, the divine office, and the sacramentals; worship, finally, which inserts the members of Christ into the heavenly current of adoration, propitiation, thanksgiving, and petition carried on by our glorified Redeemer before the throne of the Father for all eternity. 
How miraculous and great a reality, that the Mass–the culmination of liturgical and sacramental life–most authentically and powerfully “inserts the members of Christ into the heavenly current” of praise. How diligently, then, must we seek in truth to celebrate properly the Liturgy. May we continue to be nourished in the life of grace by the gift of the Eucharist. May we continue to grow in unity with Christ, the Son, so that in Him through the grace of the Spirit we may come to contemplate and love more perfectly the Most High and Sacred Lover, the Father of the Triune Divinity. Truly, may our soul soar the heavens in God’s presence as a singing bird soars through the clouds of this earthly realm. May we join with the Communion of Saints in the Heavenly Liturgy and praise the Loving Creator.
 Miller, John. “The Nature and Definition of the Liturgy.” Theological Studies. 18.3 1957: 339
 Paragraph 3. Click here for the full text.
 Paragraph 42. Click here for the full text. See also Tra le Sollecitudini, n. 2.
 cf. Address of His Holiness Benedict XVI to the Roman Curia Offering Them His Christmas Greetings, 22 Dec 2005. In this address, Benedict specifically introduces a hermeneutic of continuity or renewal, explaining that we cannot interpret Vatican II by itself without the rich history and organic developments of the Church beginning with the Apostles and ancient Fathers. Well worth a read: Click here for the full text.
 Paragraph 5 of Sacrosanctum Concilium: Constitution on the Sacred Liturgy Second Vatican Council Promulgated by His Holiness Pope Paul VI On December 4, 1963. Click here for the full text. See also n. 54: “[S]teps 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.”
 Paragraph 47 of Musicam Sacram, the Instruction On Music In The Liturgy. Sacred Congregation of Rites March 5, 1967. Click here for the full text. I also want to quote an earlier paragraph, but for the sake of space have decided to not include it in the main text of this post:
It is to be hoped that pastors of souls, musicians and the faithful will gladly accept these norms and put them into practice, uniting their efforts to attain the true purpose of sacred music, “which is the glory of God and the sanctification of the faithful.”
(a) By sacred music is understood that which, being created for the celebration of divine worship, is endowed with a certain holy sincerity of form. (Cf. St. Pius X, Motu Proprio ‘Tra le sollecitudini,’ n. 2.)
(b) The following come under the title of sacred music here: Gregorian chant, sacred polyphony in its various forms both ancient and modern, sacred music for the organ and other approved instruments, and sacred popular music, be it liturgical or simply religious. (Paragraph 4)
 Letter to the Bishops on the Minimum Repertoire of Plainchant: Voluntati Obsequens. Sacred Congregation for Divine Worship April 14, 1974. Click here for the full text.
 Third Typical Edition of the GIRM. Click here for the text online.
 Miller, John. “The Nature and Definition of the Liturgy.” Theological Studies 18.3 (1957): 356
Let me also suggest a stop by at Roma Locuta est, where Jake Tawney has written extensively on topics such as sacred music and the liturgy. “100 Years of Church Teaching on Gregorian Chant” in particular inspired me to write this post here.