In 1873, aged only fifteen, he was appointed First Soprano in the choir of that basilica,[4] and also became a regular member of the groups of soloists hired by Capocci to sing in the salons of Roman high society.

[5] His singing at such soirées was vividly described by Anna Lillie de Hegermann-Lindencrone, the American wife of the Danish Ambassador to the Holy See: Mrs Charles Bristed of New York, a recent convert to the Church of Rome, receives on Saturday evening ...

[6]In 1883, Capocci presented a special showcase for his protégé: the first performance in Italy of the oratorio Christus am Ölberge by Beethoven, in which Moreschi sang the demanding coloratura role of the Seraph.

[7] On the strength of this performance, he became known as l'Angelo di Roma, and shortly after, having been auditioned by all the members of the Sistine Chapel Choir, he was appointed First Soprano there, a post he held for the next thirty years.

[9] Moreschi's talent occasionally led to "behaviour [that] was often capricious enough to make him forget a proper professional bearing, as on the occasion after a concert when he paraded himself among the crowd like a peacock, with a long, white scarf, to be congratulated ..."[10] The Sistine Chapel Choir was run on centuries old traditional lines with a strict system of hierarchies.

This period was one of disruption within the Sistine choir's organisation as well as Catholic church music at large as the reforming movement known as Cecilianism, which had originated in Germany, began to influence Rome.

Its calls for the Church's music to return to the twin bases of Gregorian chant and the polyphony of Palestrina threatened both the repertoire and the practice of the Sistine Chapel.

Among the pontiff's first official acts was the promulgation of the motu proprio, Tra le sollecitudini ("Amidst the Cares") on St Cecilia's Day, 22 November 1903, stating: "Whenever ... it is desirable to employ the high voices of sopranos and contraltos, these parts must be taken by boys, according to the most ancient usage of the Church."

Around Easter 1914 he met Viennese musicologist Franz Haböck, author of Die Kastraten und ihre Gesangskunst (The Castrati and their Art of Singing, published in Berlin in 1927), who had planned to cast Moreschi in concerts reviving the repertoire of the great eighteenth-century castrato Farinelli.

His funeral Mass was a large and public affair in the church of San Lorenzo in Damaso, and was conducted by Perosi, who, in spite of his antipathy towards castrati, felt a "great friendship which bound them together".

Decades later, Fred Gaisberg recalled making these historic first recordings in the Vatican: "Selecting a great salon with walls covered with Titians, Raphaels, and Tintorettos, we mounted our grimy machine right in the middle of the floor.

The dated aesthetic of Moreschi's singing, involving extreme passion and a perpetual type of sob, often sounds bizarre to the modern listener, and can be misinterpreted as technical weakness or symptomatic of an aging voice.