Regarding music in the Office, the bishops said this:
To whatever extent may seem desirable, the hymns are to be restored to their original form, and whatever smacks of mythology or ill accords with Christian piety is to be removed or changed. Also, as occasion may arise, let other selections from the treasury of hymns be incorporated.
By “original form,” what would they mean? I’m assuming that hymns were sung in plainsong already, so they might mean texts. “Other selections” seems to indicate hymns from outside the tradition of the Liturgy of the Hours. That could mean pieces used for the Mass. Could it also mean non-Roman sources? Any musicologist out there who knows?