De musica sacra et sacra liturgia
21. ... c) But if for some reason a choir cannot sing one or another liturgical text according to the music printed in the liturgical books, the only permissible substitution is this: that it be sung either recto tono, i.e., on a straight tone, or set to one of the psalm tones. Organ accompaniment may be used. Typical reasons for permitting such a change are an insufficient number of singers, or their lack of musical training, or even, at times, the length of a particular rite or chant.
Musicam sacram
8. ... If, however, a choice of this kind cannot be made, and the priest or minister does not possess a voice suitable for the proper execution of the singing, he can render without singing one or more of the more difficult parts which concern him, reciting them in a loud and distinct voice.
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