(This was written on the back of a manuscript page. The info in it is pretty scrambled and ahistorical, and nobody has found the book listed as supposedly by Jerome, but it's a pretty nice couple of things to say about hymnody all the same. The translation can be found in Vol. II of The Irish Liber Hymnorum, edited by Bernard and Atkinson, and published in 1898.

A holy pope in Rome there was, exalted and excellent, whose name was Pope Clement. From him Jerome asked a description of the Psalms and hymnody. And he took to beseeching the Creator aright that night up till morning, when an angel of God came to him from heaven with the description of hymnody.

And this is what he told him:

"Whoever should recite [chant] the hymnody would be making a song of praise dear to God: for it wipes out all sins, and cleanses the powers of the body, and subdues involuntarily the lusts of the flesh; it lessens melancholy and all madness; it breaks down anger, expels Hell's angels, and gets rid of devils; it dispels the darkness of the understanding and increases holiness; it preserves health and completes good works; and it kindles a spiritual fire in the heart -- ie, the love of God in place of the love of man -- and peace between the body and the soul."

As Jerome said in the seventh chapter of The Medicine of the Soul, "O man... there is not anything more useful to thee in thy mortal state than praising God; for if thou praise God, He heals thy soul and body together. In truth, O man, inasmuch as this is thy healing, give honor to the Psalms and the hymn book...."