4 “Let them praise His Name in chorus”. What means “chorus”? Many know what a “chorus” is: nay, as we are speaking in a town, almost all know. A “chorus” is the union of singers. If we sing “in chorus,” let us sing in concord. If any one's voice is out of harmony in a chorus of singers, it offends the ear, and throwes the chorus into confusion. If the voice of one echoing discordantly troubles the harmony of them who sing, how does the discord of heresy throw into confusion the harmony of them who praise.
The whole world is now the chorus of Christ. The chorus of Christ sounds harmoniously from east to west. “Let them sing a psalm unto Him with timbrel and psaltery.” Wherefore takes he to him the “timbrel and psaltery”? That not the voice alone may praise, but the works too. When timbrel and psaltery are taken, the hands harmonize with the voice. So too do thou, whenever you sing, “Halleluia,” deal forth your bread to the hungry, clothe the naked, take in the stranger: then does not only your voice sound, but your hand sounds in harmony with it, for your deeds agree with your words.
You have taken to you an instrument, and your fingers agree with your tongue. Nor must we keep back the mystical meaning of the “timbrel and psaltery.” On the timbrel leather is stretched, on the psaltery gut is stretched; on either instrument the flesh is crucified. How well did he “sing a psalm on timbrel and psaltery,” who said, “the world is crucified unto me, and I unto the world”? This psaltery or timbrel He wishes you to take up, who loves a new song, who teaches you, saying to you, “Whosoever wills to be My disciple, let him deny himself, and take up his cross, and follow Me.” Let him not set down his psaltery, let him not set down his timbrel, let him stretch himself out on the wood, and be dried from the lust of the flesh.
The more the strings are stretched, the more sharply do they sound. The Apostle Paul then, in order that his psaltery might sound sharply, what said he? “Stretching forth unto those things which are before,” etc. He stretched himself: Christ touched him; and the sweetness of truth sounded.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)