The new album from the Sistine Chapel Choir contains a different version of Allegri's Miserere than we are used to, as this site explains:
The Deutsche Grammophon-recorded album even includes a world premiere, but it's not one you’d expect. Allegri’s Miserere may have been topping the classical charts for years, but the Sistine Chapel’s performance of the work in its original form (from the Sistine Codex of 1661) is, unbelievably, the first of its kind. Hardly anyone alive today will have heard the Miserere as Allegri originally conceived it. The version that most of us know – below, with its thrilling high C in the semichorus – is from a much later transcription.
‘The version of the Miserere that most people are familar with is based on a later transcription, most probably the one made by [former choir director] Lorenzo Perosi at the beginning of the 20th century,’ says Palombella [the current choir director]. ‘That version has the additions of many Vatican singers, but we have now recorded the true, Renaissance Miserere.
'It’s very simple, very clear, and we are singing it as authentically as possible, given what we know about the choir’s history. The soloist on the recording sings from the Salla Regia next to the Sistine Chapel, and we have also used two male cantuses [countertenors] in the semi-chorus rather than boy singers, because this is most probably how it was done in the past.’
The article also addresses the story of Mozart's secretive transcription of the music:
Allegri’s famous setting of Psalm 51 is already the stuff of legend – not least because of the widely-circulated tale about Mozart allegedly defying papal law by smuggling it out of the Vatican using memory alone. According to the story, the 14-year-old Mozart heard the Miserere during Mass on a visit to Rome in 1770 and wrote it down from memory that evening. Is it true?
‘I can believe the Mozart story is true,' says Palombella, 'but mostly because it’s very easy to transcribe the Miserere! The harmony is always the same, and it’s repeated many times. But the part about the music not being allowed to leave the Vatican is probably a myth. It is true that music was written specifically for the Sistine Chapel, and that there was a sort of copyright, but if a pope was asked to have the scores sent somewhere else, generally he accepted.’