IMPORTANT EDIT: Outside of France, Parts of England, Italy, parts of Germany I am uncertain of the involvement levels of the laity in the service, especially in smaller communities. I've been trying to dig up sources for the past couple of days, but it's hard to since I'm in-between institutions of learning. Please, if anyone can complicate my picture of congregational singing in the Mass I'd really appreciate it.

The medieval is a pretty large temporal area. The traditions of the Church changed a lot during that time. For example, the Council of Laodicea banning the members assembled within the congregation to sing (Canon 15: "No others shall sing in the Church, save only the canonical singers, who go up into the ambo and sing from a book.) but before that recitation of the psalms was definitely congregational (and I'm certain there was some exceptions).

I can say pretty confidently that by 1000 or so that most of the Canonical singing of a Mass done by the book would have been done by the layfolk (clergy and monastics) and professional singers, especially with Charlemagne's allegiance to the Roman Church and his insistence on importing contemporary Roman Music practices (and singers) into what is now modern day France. Additionally the music over time became a lot more difficult, and there became a lot more of it to mess with due to the church continuing to complicate the liturgical calendar over the years. Gregorian chant and plainchant was based off of psalm-tone formulae, but it later diverged into more complicated settings (such as incredibly melismatic and non-patterned settings of the "Alleluia" of the mass, requiring significant memorization skills that you needed music training to develop.

So definitely by 1000 it would be unlikely for the laity to sing the written out portions of the mass proper.

However, there are instances in which the laity did sing German hymns and the such in Church. However, we don't really know exactly how frequent these were sung. On the bright side, we have books and books of them. If something is valuable enough to print, it means that it had enough demand to make money off of it.

Certainly by the 15th century the music of the Mass was not sung by anything except for professionals/trained singers. The music was polyphonic, sometimes imitative, and part of a literate tradition.

Interestingly, a similar process of "becoming too difficult so professionals have to sing it" happened with Madrigals, which were originally sold as polyphonic part songs for the upper-middle class to sing at social gatherings and the such.

However, I would hesitate to say that people in Church necessarily followed a strict decorum of silence and immobility. Plenty of traditions within local communities, paired with a noticeable lack of chairs, meant at the very least there was going to be some movement (perhaps even standing up and sitting down) and on special days some local tradition might butt in to say hello.

In his book on the Protestant Reformation, Diarmaid MacColluch has this interesting anecdote about a funny local tradition within the congregation of Eichstatt

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"A small carved wooden dove of the Spirit was lowered down on the congregation through a hole in church roof-vaulting...closely followed by bucketfuls of water, and the member of the congregation most thoroughly soaked became the town's Pfingstovogel (Whitsun bird) for the coming year...the apparent irreverance was itself a symptom of how strongly the vast majority of the people felt faith int he system, and how much they could relax in it" (MacColluch, 24)

A far-cry from silent and immobile.

Herl, Joseph. Worship Wars in Early Lutheranism: Choir, Congregation, and Three Centuries of Conflict. Oxford University Press, 2008.

Seay, Albert. Music in the Medieval World. Waveland Press, 1991.

Canons of The Council of Laodicea. Trans. Henry Percival. From Nicene and Post-Nicene Fathers, Second Series, Vol. 14. Edited by Philip Schaff and Henry Wace.(Buffalo, NY: Christian Literature Publishing Co., 1900.)

McCollough, Diarmaid. The Reformation. New York, Penguin, 2005. 24.