In the April 2001 online edition of the Adoremus Bulletin, we read Part II of an article called Buried Treasure, by Susan Benofy, which poses the question, "Can the Church recover her musical heritage?"
Editor's Note: Part I of Buried Treasure traced the history of the reform of the liturgy and its music in the first half of the twentieth century. Pope Saint Pius X issued Tra Le Sollecitudini in 1903, a Motu Proprio that was the first in a series of papal and curial documents urging the active participation (participatio actuosa) of the people in the Mass, especially through the singing of Gregorian Chant. The spread of the desired reforms was slow, and the movement for reform was not without abuses. Later popes continued to encourage participatio actuosa and a greater use of chant and the rest of the treasury of sacred music at Mass. The Congregation of Rites summarized the series of documents and codified the musical reforms in a 1958 Instruction on Sacred Music and the Sacred Liturgy, De Musica Sacra. Pope Pius XII appointed a commission to study liturgical reform, and their work led to the reform of the Holy Week liturgy in the 1950s and to a simplification of the rubrics in 1960. It was in the midst of this ongoing liturgical reform that the recently elected Pope John XXIII issued his call for an Ecumenical Council in 1962.
Part II: Sacred Music in 20th Century Liturgical Reform
Contents:
Norms and precepts on music
Implementation and experts
International music organizations
Universa Laus and Father Gélineau
Post-conciliar revision
Developing the "Simple Gradual"
"Simple Gradual" approved
Simple Gradual led to "Diversity and adaptation"
The Instruction, Musicam Sacram
Preserving the treasury
Effects of Musicam Sacram

I never knew that liturgical reform actually began before Vatican II. I always thought it was a result of the Council.