Psalms 81:3

"Blow the trumpet at the new moon, on our festival day of good omen. Because it is a command for Israel, a judgment of the God of Jacob." God ordered the priests to use the trumpets. They reminded the people of the trumpets used on the mountain: when the God of all spoke on Mount Sinai, [Scripture] says, there was a loud noise of the trumpet. So when the priests used the trumpets, they reminded the people of that appearance. Consequently, they were right to command those who had been granted return and had enjoyed the divine assistance to make use of the trumpets along with the other instruments.

"Sound the trumpet" [Psalm 81:3]. This is, Loudly and boldly preach, be not affrighted! As the Prophet says in a certain place, "Cry out, and lift up as with a trumpet your voice." [Isaiah 58:1] Sound the trumpet in the beginning of the month of the trumpet. It was ordered, that in the beginning of the month there should be a sounding of the trumpet: and this even now the Jews do in bodily sort, after the spirit they understand it not. For the beginning of the month, is the new moon: the new moon, is the new life. What is the new moon? "If any, then, is in Christ, he is a new creature." [2 Corinthians 5:17] What is, "sound the trumpet in the beginning of the month of the trumpet"? With all confidence preach ye the new life, fear not the noise of the old life.

"Sound the trumpet at the beginning of the month, on the day of your notable solemnity." We must also consider that we are commanded through the instruments of the musical discipline both to play the lyre for the Lord and to observe the day of solemnity, so that every action of ours may be directed to the Lord and offered to his ears in most pleasant music, just as musical instruments are directed towards a sweet-sounding melody and coalesce smoothly into one harmony. For there is great power and delightful knowledge in that discipline, which the teachers of secular literature (as God graciously granted them to know, since he bestows everything that is useful) made able to be discerned through their theoretical writings, namely those things which in the nature of matter were previously held in secret. Therefore, the first division of this discipline is harmony, rhythm and meter. The second is a division of instruments into percussion, stringed instruments and blown instruments. The third is divided into six harmonies. The fourth is divided into fifteen tones. Thus, the power of all that most beautiful discipline is explained by such distinctions of ancient teachers; we read in the secular books that many miracles have occurred through these means. But—to omit the legendary tales of secular literature—we read that David drove a demon from Saul by his melodious harp and the divine Scripture attests that the walls of Jericho fell straightaway by the sounding of the horns, so that there can be no doubt that musical sounds often accomplish great and powerful deeds, at least if the Lord orders and permits them.