64 As we have written above, such hymns cannot be used in Solemn High Masses without the express permission of the Holy See. Nevertheless at Masses that are not sung solemnly these hymns can be a powerful aid in keeping the faithful from attending the Holy Sacrifice like dumb and idle spectators. They can help to make the faithful accompany the sacred services both mentally and vocally and to join their own piety to the prayers of the priest. This happens when these hymns are properly adapted to the individual parts of the Mass, as We rejoice to know is being done in many parts of the Catholic world.
Source: Musicae Sacrae (Vatican.va)