The Roman Rite has two forms: Ordinary, and Extraordinary. The Ordinary Form (OF) is often called “Novus Ordo”, or “Vatican II” Mass. The Extraordinary Form (EF) is often called “Tridentine Mass”.
The sacred music recommended by the Catholic Church for both forms of the Roman Rite, Ordinary and Extraordinary, have a lot in common:
- Gregorian Chant is the preferred music of the Church,
- When the Catholic liturgy calls for “necessary” music, it is always vocal and monophonic music (chant), not instrumental and/or polyphonic (several voices).
- Musical instruments, especially the organ, are welcome in both forms of the rite as a complement, a support and an enhancement of the vocal music. The human voice is however the primary “instrument” of the Liturgy.
- In practice, however, most music directors in Catholic Churches are instrumentalists, not vocalists. Their training is often stronger in the sacred repertoire for their instrument than in the documents of the Catholic liturgy. Consequently, in most parishes, the three points above this fourth one are often ignored.
If you are interested in the documents regulating the musical celebration of the Extraordinary Form of the Roman Rite (Tridentine Mass), a good place to start are pages 9 to 15 of this document : Psallite-Sapienter-A-Musicians-Guide-to-the-Extraordinary-Form-2
If the priest does not chant in a Tridentine Mass, then we have what is called “Low Mass”. The rules for singing are then different: Music-for-low-mass
(NOTE: it is illuminating to read this last document. Priests are now rarely singing. These pre-Vatican II rules of singing hymns, including in the vernacular, in lieu of the Proper, seem to have become the norm in our post-Vatican II Catholic masses.).