From Sacrosanctum Concilium:
"The treasure of sacred music is to be preserved and fostered with great care." (Nothing about throwing out the old music for new wineskins. How can you preserve and foster what you banish?)

"The Church acknowledges Gregorian chant as specially suited to the Roman liturgy: therefore, other things being equal, it should be given pride of place in liturgical services.

But other kinds of sacred music, especially polyphony, are by no means excluded from liturgical celebrations, so long as they accord with the spirit of the liturgical action, as laid down in Art. 30." (This does not occur in the vast majority of Ordinary Form parishes.)

"The texts intended to be sung must always be in conformity with Catholic doctrine; indeed they should be drawn chiefly from holy scripture and from liturgical sources."

So much of the new music contains heretical notions, banal lyrics and melodies that are like campfire songs for children, or Broadway and not solemn.