I've finally found, after many years of searching, a video showing Mass celebrated according to the third and last degree of participation at High Mass as described in De Musica Sacra (1957) under the heading "More Perfect Worship":

c) Thirdly, if those present are well trained in Gregorian chant, they can sing the parts of the Proper of the Mass. This form of participation should be carried out particularly in religious congregations and seminaries.

https://www.youtube.com/watch?v=DixSwPTrDhs

As explained on another thread, this is a group of seminarians and priests from Econe, Switzerland, on pilgrimage in Italy and celebrating Mass and Vespers in a small mountain church on the feast of St. Catherine of Siena, patroness of Italy.

ALL the responses, parts of the Ordinary and the Propers are sung by the congregation, who are seminarians and priests, led by the schola situated in the centre of the nave. It's just astonishing to hear an entire congregation singing the full Introit as Mass begins.