The charter of the CIMS read as follows:
It was the intention of these popes that all those who devote themselves to the noble art of sacred music should be in closer touch with each other and with the Holy See, and that there should be available to the Holy See some form of international institute which would be able to make known the needs of sacred music, and which would be able to assist in putting the decisions of the supreme ecclesiastical authority relating to sacred music into practice. This institute should also help missionaries to solve the difficult and important problem of sacred music in the missions, and should co-ordinate the various initiatives in this field. Finally, it was intended to promote the publication of works on sacred music and the study of the past heritage. In the Instruction on Sacred Music of 1958, Pope Pius XII, our late predecessor, advised that there should be an increase in the number of societies for the fostering of sacred music and that by their federation at the national or international level, many advantages would accrue to this science and art.
At the first general convention in 1965, the president of the CIMS, Fr. Johannes Overath said:
...if anyone were to ask me what our position is in this time after the close of the Council, I should have to say emphatically: we are not reactionaries or traditionalists; we are not fanatic progressivists who abandon tradition. When we stand for the preservation of the patrimony of sacred music in the Church, we are not mere curators of museums, because in this very letter the Holy Father has reconfirmed the words of Saint Pius X, "the spirit of piety is always promoted by the treasury of sacred music." We further believe that it is this treasury that is so important for the pastoral care of man in our century. At the same time, we stand also for the development of a new sacred music in the sense of the Vatican Council. This means especially the creation of new compositions in the vernacular languages in an artistic manner and with the characteristics of individual peoples. It means further that in this new music the entire People of God find opportunity to worship in song.
If you want to read the full proceedings, they are collected in this 316-page book. What you find here is a globe of Catholic musicians and theologians united behind the musical/liturgical goals of Vatican II. They were all the great musicians of the preconciliar period, the masters of the craft, and they saw in Vatican II the greatest opportunity for sacred music ever given to musicians, for never before had a Church Council been so explicit. There was a push for the replacing of vernacular hymnody with Latin chant, an effort to teach chant to congregations, a drive for new compositions in the polyphonic tradition, and interest in new forms of vernacular propers so that people and scholas would sing the Mass rather than just sing music at Mass.
What's interesting about this volume is that it documents the existence of the true spirit of Vatican II - regardless of what you may have heard otherwise. It also hints at the existence of another spirit alive at the time, the spirit of the culture at large that would eventually overwhelm the attempt to guide the aesthetic of the liturgy in the true spirit of the Council. It makes for sad reading but it is good to know the history here.