Thomas Aquinas

“ 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men's minds towards God by means of preaching and teaching, ought not to be instant in singing, lest thereby they be withdrawn from greater things. ”

“ Me with their lips, but their heart is far from Me." Therefore the praise of God lies in the heart rather than on the lips.
Objection 3: Further, men are praised with the lips that they may be encouraged to do better: since just as being praised makes the wicked proud, so does it incite the good to better things. ”

“ For the Apostle says (Col. 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 Eph. 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. ”

“ For such like musical instruments move the soul to pleasure rather than create a good disposition within it. In 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. ”

“ 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. ”