Catechism of the Catholic Church §1157 — excerpt
Song and music fulfill their function as signs in a manner all the more significant when they are "more closely connected . . with the liturgical action," according to three principal criteria: beauty expressive of prayer, the unanimous participation of the assembly at the designated moments, and the solemn character of the celebration.
This translation is under copyright, so only the cited words are quoted here. Follow the link for the full text. Translation: English translation of the Catechism of the Catholic Church, 2nd ed. (Libreria Editrice Vaticana), as posted at vatican.va.
Also cites: SC 112 # 3 · SC 112 · St. Augustine, Conf. 9, 6, 14: PL 32, 769-770