• I never noticed before, but the GIRM has a footnote (55) in ¶ 48, in reference to the list of appropriate music for the Entrance. It cites ¶ 50 of Pope Saint John Paul II's Apostolic Letter Dies Domini and seems to contain guidance as to what constitutes "a suitable liturgical song."

    50. Given the nature of Sunday Mass and its importance in the lives of the faithful, it must be prepared with special care. In ways dictated by pastoral experience and local custom in keeping with liturgical norms, efforts must be made to ensure that the celebration has the festive character appropriate to the day commemorating the Lord's Resurrection. To this end, it is important to devote attention to the songs used by the assembly, since singing is a particularly apt way to express a joyful heart, accentuating the solemnity of the celebration and fostering the sense of a common faith and a shared love. Care must be taken to ensure the quality, both of the texts and of the melodies, so that what is proposed today as new and creative will conform to liturgical requirements and be worthy of the Church's tradition which, in the field of sacred music, boasts a priceless heritage.