Father Z has this well-put:
Holy Church asks for full, conscious and active participation at every Holy Mass, regardless of the form, whether the Novus Ordo or the more traditional, Extraordinary Form.
However, this sort of participation has often been misunderstood. For decades the dominant view of active participation has been that for everyone to be "actively participating" everyone has to be singing every world, carrying stuff around and doing things.
Yeah - like, everyone's got to have their hands on something: a hymnal, a collection basket, a pyx, a Lectionary, someone else's hand, a few bucks of someone else's money, whatever it may be.
Or that everyone has to sing every little thing that's got music to it. I grew up in one parish where the people even joined in the chanting of the Per Ipsum (until the pastor was corrected and had to pass the correction to his people).
Now, while the good Father continues about how this is applied to the Extraordinary Form of the Mass (btw, St. John Cantius is holding an Extraordinary Form workshop 8/25-29/08), the same holds true about the Ordinary Form as well. Every now and then it's good to just park your butt and open your ears and listen. So what if the choir sings a motet or an anthem at the Offertory or at Communion. Listen, just like listening to the Gospel or the Homily. Let the word of God enter through your ears and be on your minds and your hearts. That, too, is active participation.
To everything, there is a time.
A time to live, a time to die.
A time to do, sing, pray out loud,
And a time to just sit, relax, and listen,
and pray silently.