102. It is the psalmist’s place to sing the Psalm or other biblical canticle to be found between the readings. To carry out this function correctly, it is necessary for the psalmist to be accomplished in the art of singing Psalms and have a facility in public speaking and elocution.
What is the function of the choir? Simple: to sing its own parts and to foster the singing of others.
103. Among the faithful, the schola cantorum or choir exercises its own liturgical function, its place being to take care that the parts proper to it, in keeping with the different genres of chant, are properly carried out and to foster the active participation of the faithful by means of the singing.[cf. Musicam Sacram 19] What is said about the schola cantorum also applies, with due regard for the relevant norms, to other musicians, and especially the organist.
The organist, too, and any other instrumentalist or singer exist mainly to render their own parts of the Mass and to foster congregational song.
104. It is fitting that there be a cantor or a choir director to direct and support the people’s singing. Indeed, when there is no choir, it is up to the cantor to direct the different chants, with the people taking the part proper to them.[cf. Musicam Sacram 21]
Without requiring a choir at every Mass (a difficult and demanding ministry stance, but not impossible) the Church gives an adequate set of preferences. If you don’t have a big choir, a small one will do. If you don’t have a small one, then a few singers. If not those singers, then a single cantor. It sets the bar for improvement just about everywhere. Speaking for my personal goals, I’ve always striven for a choir at every Sunday and holy day Mass. I haven’t always reached that goal, and I’ve worked for a pastor or parish or two who advocated the approach of all-eggs-in-one-basket. What about your parish?