The encyclical “Tra le Sollecitudini (Instruction on Sacred Music)” was written in 1903
Does this still apply?
I’m not familiar with the document, but I note that it is a
motu proprio rather than an encyclical. (Typically the former deal with canonical or procedural matters, and the latter with doctrinal issues).
A
motu proprio or any other canonical or legal document usually remains in force until it is superceded by other instructions. I would guess that inasmuch as there have been numerous further statements on the sacred liturgy, not least
Sacrosanctum Concilium in 1963, its provisions have now been abrogated (that is, no longer apply). Note for instance that in III.7 of
Tra le Sollecitudini it states:
‘The language proper to the Roman Church is Latin. Hence it is forbidden to sing anything whatever in the vernacular in solemn liturgical functions – much more to sing in the vernacular the variable or common parts of the Mass and Office.’
That has certainly been superceded by other canonical instructions - for better or worse, depending on your liturgical preferences.
Although partial abrogation of a document does not mean total abrogation in and of itself - that is, just because one element no longer applies doesn’t mean that the whole document doesn’t apply, unless its provisions have been specifically abrogated - the specific instructions on music have most likely been superceded by the specific instructions on music in later documents.
I’m happy to be corrected by any expert on this subject, of course. Best wishes.