Secondary source - St. Cecilia Schola Cantorum in Auburn, AL. Plus, our choirmaster said it and AFAIK he is dead on about musical matters . . .
He said what, exactly?
That Pius X mandated the Italianate/ecclesiastical pronunciation in Tra le Sollecitudini? Well, he couldn't have said that, unless his memory was fuzzy.
That Pius X mandated the Italianate/ecclesiastical pronunciation? If that is what he said, does he have a source document?