In fact, in my six years of being a priest, and having visited a wide number of Catholic parishes in different states, I would say that about one out of fifty has a decent amount of participation from the people in the music. Although the intent was to increase participation, the result has been just the opposite; it has decreased (drastically). The rubrics for the Mass say that "polyphony" should "foster participation" of the people -- does it actually do so today? What went wrong? I am going to take a stab in the dark and say that the massive proliferation of available music (pushed by the motivation to have "more options") has actually smothered the whole effort.
Consider this: we all know that the large majority of people in the pews do not have much, if any, training in singing. This is not an insult, it is a fact; just like the fact that the vast majority of people in the pews do not have much, if any, training in historic Catholic architecture. With that in mind, we have to consider also that when we are dealing with music, it is not like something that everyone can be anonymous with. If someone is a poor singer, then he is very unlikely even to open his mouth during a hymn; he will just stand there and stare off into space--effectively being prevented from active participation by something that was intended to include him. As a result, the more one tunes out during the music, the more he will also tune out during the rest of the Mass (!).
One of the reasons that this "tuning out" during the music happens is because unless a parish is using a collection of only about fifteen or twenty hymns and rotating through them each week (and I know of no single parish that would even consider doing something like this -- music ministers would say "that is far too monotonous"), most people will not know many of the hymns that are being sung. Add to this the fact that whoever chooses the music in these parishes is probably choosing "new and different" hymns on a regular basis, and they want to "give variety", what they are actually giving is something that is barely possible to learn before the hymn is finished (after all, the entire congregation does not show up for practice before Mass).
How do I know this? I have had multiple "music ministers" tell me that it is rare for more than about 10% of the people at Mass actually to join in the singing at the parishes they serve. When, however, do we see them joining in? In the parts that are repeated each week (the Sursum Corda, the Alleluia, the Agnus Dei, etc.). When the people are given time to learn something (or at least have heard it repeated for weeks) then they become familiar with it and gladly join in. I see it regularly: a new hymn is chosen by the song leader, the musician plays it well (he or she had time to practice it), and the people will mostly not even look at the hymnal, while a few softly mumble out the words trying to figure out just how the tune goes (they usually "get it" just in time for the last stanza to end).
What is the solution? As is so often the case -- go back to what the Church used to do. Gregorian chant is really not that hard to learn (as the Church said at Vatican II: chants are "always to be preferred" over hymns!). If you give the people a few weeks to catch on (and stop using a repertoire of hymns that is so vast that only those who can read music well can follow it (which is about 5% of the parishioners)) then they will learn them and find them easier to learn than hymns. In the forms we use at St. George Catholic Church there are only a few basic tunes that are used and the words to the chants change based on the Mass (we repeat the forms for the Gloria, the Kyrie, etc.). We have about 95% participation in the chants on an average Sunday (and do not use any hymns during the Mass).
Furthermore, the chants given in the Missal allow us to sing about the full breadth of our faith and not limit it to the syrupy content in many hymns. I do not even want to go into the vapid and empty words of most of those musical pieces called "praise tunes" (maybe "nursery rhymes" would be better). When was the last time that a "praise tune" or a hymn sang words like:
Haste thee, O God, to deliver me; make haste to help me, O Lord: let mine enemies be ashamed and confounded that seek after my soul. Let them be turned backward and put to confusion: that wish me evilThis is what was sung for the Introit (the opening chant) a few weeks ago (the words come from Psalm 70)? Furthermore this helps to prevent the one choosing the music from sticking to the "nice stuff" (which keeps the faithful from numerous aspects of the faith).
Will this change? Will parishes choose to use exclusively the proper chants that are listed in the Missal and abandon the hymns that have caused so much disconnect from the Mass? I believe that it will happen; someday. Yet, it will only happen when we turn ourselves to a deeper commitment to reverence in the worship of God. It will only happen when we realize that any sense of entertainment (which is the root motivation in most musical choices in Catholic parishes) in the Mass is to be condemned as an attempt to please self more than please God. It will only happen when we turn our hearts to the Lord and worship Him above all else.