The 1955 encyclical is here:
It is the duty of all those to whom Christ the Lord has entrusted the task of guarding and dispensing the Church’s riches to preserve this precious treasure of Gregorian chant diligently and to impart it generously to the Christian people…. And if in Catholic churches throughout the entire world Gregorian chant sounds forth without corruption or diminution, the chant itself, like the sacred Roman liturgy, will have a characteristic of universality, so that the faithful, wherever they may be, will hear music that is familiar to them and a part of their own home. In this way they may experience, with much spiritual consolation, the wonderful unity of the Church. This is one of the most important reasons why the Church so greatly desires that the Gregorian chant traditionally associated with the Latin words of the sacred liturgy be used.