Shiurim from R' Eli Pick on עניני יו"כ:
A shiur from the Chabad Research Unit on the halachic and mystical aspects of כל נדרי:
A famous speaker once said that the (Ashkenazi (!)) tune (perhaps the text as well?) for כל נדרי originates from the Marranos in Spain, who used כל נדרי as a method of absolving themselves before 'ה from their vows to the church.
This assertion is absurd, for multiple reasons:
1) The text of כל נדרי dates back to the Geonic period, long before the Marranos (see, for example, מחזור עליות אליהו יו"כ pg. 34)
2) The אשכנזי tune for כל נדרי has strong German tones, the likes of which one might hear in the music of Beethoven; most likely, it is part of the ניגוני מסיני composed by the מהרם of Rothenburg and his תלמיד the מהריל
3) The function of כל נדרי is not to absolve past vows - this is accomplished by התרת נדרים - but rather to declare all future vows null and void
4) Contrary to prevalent popular mythology, many of the Marranos were largely ignorant of anything Jewish; many of them were not even told of their Jewish identity until they were teenagers, simply because anything that smacked remotely of Judaism, or "Judaizing", was liable to be found out by the Inquisition and their network of informants ימח שמם וזכרם and, as such, any underground Jewish life that may, perhaps, have existed immediately following the expulsion quickly ceased. To say that, of all traditions, כל נדרי was kept alive - despite all Jewish practice being strictly illegal on pain of torture and often death - is a big stretch, to say the least.
5) In fact, the Spanish-Portuguese tune for כל נדרי actually sounds like this:
כל נדרי [JC]