In this article, in the section entitled The “Great” Aleinu History, the author writes that a particular melody used in Jewish High Holiday prayers was added to the French Church Mass in the late 12th century by French crusaders, and can be heard to this very day.
You can play the melody by clicking on the player under the heading "Amida (avos and Gevuros)" immediately below the previously mentioned section in the article or directly play the mp3 here (both are the correct melody, although to different words from a different part of the prayer service).
My question is, is it true that this melody can still be heard in the mass to this very day? What part of the mass is it used for? Are there any audio recordings of it?
Also, from the article linked in caseyr547's answer below:
Another report adds that the Gentiles 'henceforth used the chant in their church,' and musicologist Eric Werner did in fact locate the melody in the Sanctus of the Ninth Mass of the Virgin.