61. After the First Reading follows the Responsorial Psalm, which is an integral part of the Liturgy of the Word and which has great liturgical and pastoral importance, since it fosters meditation on the Word of God.
By “pastoral” importance, I would say this includes the spiritual aspect of the life of faith. This would be one area where I see post-conciliar liturgy as extremely fruitful: placing the words of the psalms on the lips of everyday believers.
The Responsorial Psalm should correspond to each reading and should usually be taken from the Lectionary.
In the instances when the psalm text is not from the Lectionary, there is a presumption that the choice corresponds to the readings–not just the one preceding it.
It is preferable for the Responsorial Psalm to be sung, at least as far as the people’s response is concerned. Hence the psalmist, or cantor of the Psalm, sings the Psalm verses at the ambo or another suitable place, while the whole congregation sits and listens, normally taking part by means of the response, except when the Psalm is sung straight through, that is, without a response. However, in order that the people may be able to sing the Psalm response more easily, texts of some responses and Psalms have been chosen for the different times of the year or for the different categories of Saints. These may be used instead of the text corresponding to the reading whenever the Psalm is sung. If the Psalm cannot be sung, then it should be recited in a way that is particularly suited to fostering meditation on the Word of God.
This piece concludes the universal legislation on the Psalm. Note the first preference for the ambo. Note also the acceptability of a through-sung psalm, or one of the common texts provided in the Lectionary for Mass.
In the Dioceses of the United States of America, instead of the Psalm assigned in the Lectionary, there may be sung either the Responsorial Gradual from the Graduale Romanum, or the Responsorial Psalm or the Alleluia Psalm from the Graduale Simplex, as described in these books, or an antiphon and Psalm from another collection of Psalms and antiphons, including Psalms arranged in metrical form, providing that they have been approved by the Conference of Bishops or the Diocesan Bishop. Songs or hymns may not be used in place of the Responsorial Psalm.
Here in the US, there is a wide variety of permitted sources: the Lectionary psalm(s) in any musical form. Permission for a metrical psalm implies a paraphrase of the psalm text. It seems open that if one uses the psalm of the day, there would be no need to select from a conference-approved source. And of course, such a resource currently does not exist.
That last sentence may have a few problems in context. I would presume it refers to a source text outside of the Psalter, and not the genre of the music. If anyone is aware of any official clarifications beyond a diocesan level, feel free to offer them with links. Otherwise, discuss away.