Today, as yesterday, musicians, composers, liturgical chapel cantors, church organists and instrumentalists .... should be especially conscious of the fact that each of their creations or interpretations cannot escape the requirement of being a work that is inspired, appropriate and attentive to aesthetic dignity, transformed into a prayer of worship when, in the course of the liturgy, it expresses the mystery of faith in sound.
It's attributed to JPII "in one of his letters" in a paper by NAJI HAKIM titled "Music in the Catholic liturgy in France at the end of the 20th century" (ref: http://www.najihakim.com/writings/musiqliturgifr-e.html
The paper itself is quite depressing: yet another "professional" musician who thinks that the pipe organ is the be-all-and-end-all of church music, doesn't think that active participation requires more than just listening, and thinks that musical activity should NOT be the realm of everyone.
But the quote above can be interpreted in a different way: it's saying that in by the nature of liturgy, musical pieces that are
- inspired (ie motivated by Godly thoughts about God or sacred topics)
- appropriate (ie fits the situation it is played in - which includes the age, culture, etc of the people present)
- attentive to aesthetic dignity (ie match the cultural expectations of "beauty" that the listeners have)
are changed so that they express faith in sound.
I'd take that even further: various musical forms with heavy percussion or a strong emphasis on rhythm (think tribal drumming or Irish music) both require and develop a high level unity.
And the paper makes one interesting point:
"... today on a large scale within the French Catholic cultural landscape - musical art inspired by Christianity has deserted the liturgy and taken refuge in concerts or in recordings"
That's by no means uniquely French: all over the world, people with "high art" musical tastes and faiths that reject all "liturgical trappings" or no faith at all - see no irony in going to concert performance of Masses, Requiems, Magnificats and the like.
It seems to me that Christian artists have one of the best chances of evangelising a very difficult-to-reach group - maybe just maybe the work that they put into the concert hall actually has as much importance as any church-sanctioned liturgy.