Are we even supposed to sing in the Traditional Latin Mass (Tridentine Mass) ? St Pope Pius X, in his 1903 Motu Proprio, was unambiguous. His answer is “YES!”

The ancient traditional Gregorian Chant must, therefore, in a large measure be restored to the functions of public worship, and the fact must be accepted by all that an ecclesiastical function loses none of its solemnity when accompanied by this music alone.
Special efforts are to be made to restore the use of the Gregorian Chant by the people, so that the faithful may again take a more active part in the ecclesiastical offices, as was the case in ancient times.

Tra le solicitudine – St Pope Pius X, 1903 – Paragraph 3

To explore the musical celebration of the Extraordinary Form of the Roman Rite (Tridentine Mass), it is important to understand the difference between liturgical singing (= singing the Mass) and devotional singing ( = singing at Mass). The pre-1962 rubrics thus differentiate what must be sung at High Mass (liturgical singing) and what may be sung at Low Mass (devotional singing).

• Q & A on music at High Mass
• Q & A on music at Low Mass

Liturgical chant (singing the Mass) requires more practice and discipline than devotional singing. The changes that happened in church music after Vatican II were the triumph of devotional singing over liturgical chant. The “Novus Ordo” Mass seen in most US churches today is just the continuation of the pre-1962 habit of singing devotions at Low Mass (spoken Mass). Singing the Tridentine Mass also requires a working knowledge of the 1962 Liturgical Calendar. But accepting the Roman Gradual as our guide is also much more rewarding (see Questions? or Resumé). The sung Latin mass defined in the Roman Gradual is the result of a fifteen-century-long-tradition of singing the psalms in the Christian liturgy (read “towards a definition of liturgical chant” for more details). To start a daily practice to build your breath support and be able to sing the Mass, visit “Chanting the psalms, before you start“.

These are sum-ups of these 1958 instructions, worth a read: de musica sacra et sacra liturgia

Finally, pages 9 to 15 of this document :  Psallite-Sapienter-A-Musicians-Guide-to-the-Extraordinary-Form-2

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