Objection 1. It would seem that God should not be praised with song. For the Apostle says (Colossians 3:16): "Teaching and admonishing one another in psalms, hymns and spiritual canticles." Now we should employ nothing in the divine worship, save what is delivered to us on the authority of Scripture. Therefore it would seem that, in praising God, we should employ, not corporal but spiritual canticles.
Objection 2. Further, Jerome in his commentary on Ephesians 5:19, "Singing and making melody in your hearts to the Lord," says: "Listen, young men whose duty it is to recite the office in church: God is to be sung not with the voice but with the heart. Nor should you, like play-actors, ease your throat and jaws with medicaments, and make the church resound with theatrical measures and airs." Therefore God should not be praised with song.
Objection 3. Further, the praise of God is competent to little and great, according to Apocalypse 14, "Give praise to our God, all ye His servants; and you that fear Him, little and great." But the great, who are in the church, ought not to sing: for Gregory says (Regist. iv, ep. 44): "I hereby ordain that in this See the ministers of the sacred altar must not sing" (Cf. Decret., dist. xcii., cap. On sancta Romana Ecclesia). Therefore singing is unsuitable to the divine praises.
Objection 4. Further, in the Old Law God was praised with musical instruments and human song, according to Psalm 32:2-3: "Give praise to the Lord on the harp, sing to Him with the psaltery, the instrument of ten strings. Sing to Him a new canticle." But the Church does not make use of musical instruments such as harps and psalteries, in the divine praises, for fear of seeming to imitate the Jews. Therefore in like manner neither should song be used in the divine praises.
Objection 5. Further, the praise of the heart is more important than the praise of the lips. But the praise of the heart is hindered by singing, both because the attention of the singers is distracted from the consideration of what they are singing, so long as they give all their attention to the chant, and because others are less able to understand the thing that are sung than if they were recited without chant. Therefore chants should not be employed in the divine praises.
On the contrary, Blessed Ambrose established singing in the Church of Milan, a Augustine relates (Confess. ix).
I answer that, As stated above (Article 1), the praise of the voice is necessary in order to arouse man's devotion towards God. Wherefore whatever is useful in conducing to this result is becomingly adopted in the divine praises. Now it is evident that the human soul is moved in various ways according to various melodies of sound, as the Philosopher state (Polit. viii, 5), and also Boethius (De Musica, prologue). Hence the use of music in the divine praises is a salutary institution, that the souls of the faint-hearted may be the more incited to devotion. Wherefore Augustine say (Confess. x, 33): "I am inclined to approve of the usage of singing in the church, that so by the delight of the ears the faint-hearted may rise to the feeling of devotion": and he says of himself (Confess. ix, 6): "I wept in Thy hymns and canticles, touched to the quick by the voices of Thy sweet-attuned Church."
Reply to Objection 1. The name of spiritual canticle may be given not only to those that are sung inwardly in spirit, but also to those that are sung outwardly with the lips, inasmuch as such like canticles arouse spiritual devotion.
Reply to Objection 2. Jerome does not absolutely condemn singing, but reproves those who sing theatrically in church not in order to arouse devotion, but in order to show off, or to provoke pleasure. Hence Augustine says (Confess. x, 33): "When it befalls me to be more moved by the voice than by the words sung, I confess to have sinned penally, and then had rather not hear the singer."
Reply to Objection 3. To arouse men to devotion by teaching and preaching is a more excellent way than by singing. Wherefore deacons and prelates, whom it becomes to incite men's minds towards God by means of preaching and teaching, ought not to be instant in singing, lest thereby they be withdrawn from greater things. Hence Gregory says (Regist. iv, ep. 44): "It is a most discreditable custom for those who have been raised to the diaconate to serve as choristers, for it behooves them to give their whole time to the duty of preaching and to taking charge of the alms."
Reply to Objection 4. As the Philosopher says (Polit. viii, 6), "Teaching should not be accompanied with a flute or any artificial instrument such as the harp or anything else of this kind: but only with such things as make good hearers." For such like musical instruments move the soul to pleasure rather than create a good disposition within it. On the Old Testament instruments of this description were employed, both because the people were more coarse and carnal--so that they needed to be aroused by such instruments as also by earthly promises--and because these material instruments were figures of something else.
Reply to Objection 5. The soul is distracted from that which is sung by a chant that is employed for the purpose of giving pleasure. But if the singer chant for the sake of devotion, he pays more attention to what he says, both because he lingers more thereon, and because, as Augustine remarks (Confess. x, 33), "each affection of our spirit, according to its variety, has its own appropriate measure in the voice, and singing, by some hidden correspondence wherewith it is stirred." The same applies to the hearers, for even if some of them understand not what is sung, yet they understand why it is sung, namely, for God's glory: and this is enough to arouse their devotion.
Source: Summa Theologica (New Advent)