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Attached is a booklet in PDF format with Pope St Pius X's Motu Proprio on sacred music Tra Le Sollecitudini (1903), including his accompanying letter to Pietro Cardinal Respighi, Vicar of Rome, followed by the text of perhaps a little-known instruction from Cardinal Respighi (1912) which indicates the will of the reigning Pontiff.

Cardinal Respighi was Vicar of Rome from 1900 until his death in 1913:
Pietro Respighi - Wikipedia

His letter starts on page 20. On page 22 we read: "In order to facilitate so important a work, it has seemed good to us to give a few practical rules to which by order of the Holy Father, all persons whatsoever who have to do with the performance of music in the Churches and Chapels in Rome are required to give ear".

Immediately follows: "The best ecclesiastical traditions demand that the whole assembly of the faithful should join in the singing at all liturgical functions, by executing the parts of the text which are assigned to the choir, and that a special Schola Cantorum should alternate with the people, undertaking the more richly melodious parts, which should be strictly reserved to them.

Further on, under 'Rules for Superiors of Churches' on pp 26 and 27 we read: "...they should should explain to the people... inviting the faithful to cooperate in this matter especially by taking a more active part in the sacred functions, by singing the Kyrie Eleison, the Gloria, etc., ..."

This is a good lesson on how careful we must be in judging on such matters. It would seem to vindicate Archbishop Lefebvre (and the sermon given by Bishop Tissier which I posted in another thread) as also the Popes following Pope Pius X up to Pius XII who have been harshly criticised in this and other matters by some contemporary scholars who too readily discard the liturgical directives of the Vicar of Christ as being the deviations of a hijacked Liturgical Movement.

I guess my first thought was, “If congregational singing was so traditional, then why did the people need to be told they should be doing it?”

It reminded me of St. Pius X’s exhortation to receive daily communion: It might be a good thing for some, but it definitely wasn’t traditional.

According to Carol Byrne's book Born of Revolution, A Misconceived Liturgical Movement, the original Motu Proprio (in Latin) by Pius X NEVER indicated the people should sing.  The word "active" as in active participation never appears in the original Latin but does appear in the Italian translation dated the same day.
To understand what His Holiness wrote we need someone familiar with Latin to read the original.  What we have access to in various languages was compromised on day one.
I'm only on chapter three but carol obviously did extensive research.

Yes. So says Dr Carol Byrne. That is my point. But let us forget about the word 'active' and just read what is said here by the Cardinal Vicar of Rome, the one entrusted by Pope St Pius X to put his Motu Proprio into effect. He is here laying down the rules to the clergy of Rome which "BY ORDER OF THE HOLY FATHER", "ALL PERSONS", "ARE REQUIRED TO GIVE EAR". Does it not mean something? I don't think we need to be Latinists to make sense of this. Sean says it is not traditional. But Cardinal Respighi, in the name of the Holy Father St Pius X, in the Pope's backyard says "THE VERY BEST ECCLESIASTICAL TRADITIONS DEMAND THAT THE WHOLE ASSEMBLY OF THE FAITHFUL SHOULD JOIN IN THE SINGING". It makes a mockery of the Church to refuse this. St Pius X, Pius XI, Pius XII, Archbishop Lefebvre, the great prelate raised up by God to preserve Tradition in this crisis. What is left of the Magisterium if we set ourselves up to judge these great men of God and the Church? "My research", that is what is left...