ChapterV-II. Singing in the Office
267. In the rubrics and norms of this Instruction, the words "say .. .. recite," etc., are to be understood to refer to either singing or recitation, in the light of the principles that follow.
268. "The sung celebration of the divine office is more in keeping with the nature of this prayer and a mark of both higher solemnity and closer union of hearts in offering praise to God. . . . Therefore the singing of the office is earnestly recommended to those who carry out the office in choir or in common." 
269. The declarations of Vatican Council II on liturgical singing apply to all liturgical services but in a special way to the liturgy of the hours.  Though every part of it has been revised in such a way that all may be fruitfully recited even by individuals, many of these parts are lyrical in form and do not yield their fuller meaning unless they are sung, especially the psalms, canticles, hymns, and responsories.
270. Hence, in celebrating the liturgy singing is not to be regarded as an embellishment superimposed on prayer; rather, it wells up from the depths of a soul intent on prayer and the praise of God and reveals in a full and complete way the community nature of Christian worship.
Christian communities of all kinds seeking to use this form of prayer as frequently as possible are to be commended. Clerics and religious, as well as all the people of God, must be trained by suitable catechesis and practice to join together in singing the hours in a spirit of joy, especially on Sundays and holydays. But it is no easy task to sing the entire office; nor is the Church's praise to be considered either by origin or by nature the exclusive possession of clerics and monks but the
property of the whole Christian community. Therefore several principles must be kept simultaneously in mind if the sung celebration of the liturgy of the hours is to be performed correctly and to stand out in its true nature and splendor.
271. It is particularly appropriate that there be singing at least on Sundays and holydays, so that the different degrees of solemnity will thus come to be recognized.
272. It is the same with the hours: all are not of equal importance; thus it is desirable that those that are the true hinges of the office, that is, morning prayer and evening prayer, should receive greater prominence through the use of singing.
273. A celebration with singing throughout is commendable, provided it has artistic and spiritual excellence; but it may be useful on occasion to apply the principle of "progressive solemnity." There are practical reasons for this, as well as the fact that in this way the various elements of liturgical celebration are not treated indiscriminately, but each can again be given its connatural meaning and genuine function. The liturgy of the hours is then not seen as a beautiful memorial of the past demanding intact preservation as an object of admiration; rather it is seen as open to constantly new forms of life and growth and to being the unmistakable sign of a community's vibrant vitality.
The principle of "progressive solemnity" therefore is one that recognizes several intermediate stages between singing the office in full and just reciting all the parts. Its application offers the possibility of a rich and pleasing variety. The criteria are the particular day or hour being celebrated, the character of the individual elements comprising the office, the size and composition of the community, as well as the number of singers available in the circumstances.
With this increased range of variation, it is possible for the public praise of the Church to be sung more frequently than formerly and to be adapted in a variety of ways to different circumstances. There is also great hope that new ways and expressions of public worship may be found for our own age, as has clearly always happened in the life of the Church.
274. For liturgical celebrations sung in Latin, Gregorian chant, as the music proper to the Roman liturgy, should have pride of place, all other things being equal.  Nevertheless, "the Church does not exclude any type of sacred music from liturgical services as long as the music matches the spirit of the service itself and the character of the individual parts and is not a hindrance to the required active participation of the people." At a sung office, if a melody is not available for the given antiphon, another antiphon should be taken from those in the repertoire, provided it is suitable in terms of nos. 113 and 121-125.
275. Since the liturgy of the hours may be celebrated in the vernacular, "appropriate measures are to be taken to prepare melodies for use in the vernacular singing of the divine office." 
276. But it is permissible to sing the various parts in different languages at one and the same celebration. 
277. The decision on which parts to choose for singing follows from the authentic structure of a liturgical celebration. This demands that the significance and function of each part and of singing should be fully respected. Some parts by their nature call for singing:  in particular, acclamations, responses to the greetings of priest and ministers, responses in litanies, also antiphons and psalms, the verses and reprises in responsories, hymns and canticles. 
278. Clearly the psalms are closely bound up with music (see nos. 103-120), as both Jewish and Christian tradition confirm. In fact a complete understanding of many of the psalms is greatly assisted by singing them or at least not losing sight of their poetic and musical character. Accordingly, whenever possible singing the psalms should have preference, at least for the major days and hours and in view of the character of the psalms themselves.
279. The different ways of reciting the psalms have been described in nos. 121-123. Varying these ways should depend not so much on external circumstances as on the different genres of the psalms to be recited in the same celebration. Thus the wisdom psalms and the narrative psalms are perhaps better listened to, whereas psalms of praise and thanksgiving are of their nature designed for singing in common. The main consideration is to ensure that the celebration is not too inflexible or elaborate nor concerned merely with formal observance of rules, but that it matches the reality of what is celebrated. The primary aim must be to inspire hearts with a desire for genuine prayer and to show that the celebration of God's praise is a thing of joy (see Ps 147).
280. Even when the hours are recited, hymns can nourish prayer, provided they have doctrinal and literary excellence; but of their nature they are designed for singing and so, as far as possible, at a celebration in common they should be sung.
281. The short responsory after the reading at morning prayer and evening prayer (see no. 49) is of its nature designed for singing and indeed for congregational singing.
282. The responsories following the readings in the office of readings by their very nature and function also call for their being sung. In the plan of the office, however, they are composed in such a way that they retain their power even in individual and private recitation. Responsories set to simpler melodies can be sung more frequently than those responsories drawn from the traditional liturgical books.
283. The longer readings and the short readings are not of themselves designed for singing. When they are proclaimed, great care should be taken that the reading is dignified, clear, and distinct and that it is really audible and fully intelligible for all. The only acceptable melody for a reading is therefore one that best ensures the hearing of the words and the understanding of the text.
284. Texts that are said only by the person presiding, such as the concluding prayer, can be sung gracefully and appropriately, especially in Latin. This, however, will be more difficult in some languages, unless singing makes the texts more clearly audible for all.