Musicae Sacrae §66

⁶⁶ Hence We can do no less than urge you, venerable brethren, to foster and promote diligently popular religious singing of this kind in the dioceses entrusted to you. There is among you no lack of experts in this field to gather hymns of this sort into one collection, where this has not already been done, so that all of the faithful can learn them more easily, memorize them and sing them correctly.