” Not addressed is the matter of including musical instruments (other than the human voice) in the liturgy. Some Orthodox and a few non liturgical traditions still do not allow it.”
From Tra le Sollecitudini (linked above):
” 15. Although the music proper to the Church is purely vocal music, music with the accompaniment of the organ is also permitted. In some special cases, within due limits and with proper safeguards, other instruments may be allowed, but never without the special permission of the Ordinary, according to prescriptions of the Caeremoniale Episcoporum.
16. As the singing should always have the principal place, the organ or other instruments should merely sustain and never oppress it.”
(Also, that document also prohibited piano, cymbals, drums, etc)
I am curious how the organ got included.