"Recorded live during Mass on 9/14/08 and sung by Les Choristes, the vocal quartet in residence at the French National Church in San Francisco, CA (Steven Olbash, director)."
Vexilla regis prodeunt is the Vespers hymn for the week of Lent 5 and Holy Week; you can find the words here or here. Guilliame Dufay was a Franco-Flemish composer of the early Renaissance, and lived and composed during the 15th century.
Here's a (sadly) short version of Palestrina's take on Vexilla Regis:
Bruckner got into the act, too:
But here's something interesting: a video of the singing of Vexilla Regis in procession:
At the YouTube page, you find, in French, an explanation: the music is Vexilla Regis (Anthoine de Bertrand, 1530-1581) and the video is from the "Mass of the Presanctified on Holy Friday" at Saint Eugène à Paris. In the comments, there's this:
On entend le son des crécelles. J'ai un souvenir d'enfant de choeur. Nous aimions les offices de la semaine sainte, en particulier pour actionner la crécelle. On l'utilisait dès le jeudi saint après le gloria, où les enfants avaient sonné dans le choeur avec une vigueur particulière. La schola continuait a capella et à l'élévation la cloche était remplacée par la crécelle.
That is more or less this: "You can hear the sound of rattles. I have a memory from when I was a choir boy. We liked the offices of Holy Week, and particularly to activate the rattle. It was used on Maundy Thursday after the Gloria, and the choir children used it with special vigor. The choir continued a capella and at the elevation the bell was replaced by rattle."
In other words: the sound is the French version of a crotalus. Here's the Wikipedia page for crécelle - and the text, with translation:
Une crécelle est un instrument de musique idiophone datant du Moyen Âge, aussi appelée brouan et répandue aujourd'hui encore partout en Europe. De conception et d'utilisation simples, elle est un instrument populaire mais aussi un jouet pour les enfants.
Grâce au bruit puissant qu'elle émet, elle était aussi utilisée au Québec par les femmes des agriculteurs pour appeler leur mari au champ, avant la mécanisation de l'agriculture.- Dans la liturgie catholique, avant Vatican II, maniée dans les rues par les enfants de chœur, elle annonçait les offices durant le triduum pascal en remplacement des cloches.
On l'utilisait aussi afin d'avertir du passage de personnes infectieuse, atteintes de maladies redoutées au Moyen Âge : la lèpre, la peste.
A rattle is a percussive musical instrument from the Middle Ages, also known as the brouan, and still widely used throughtout Europe. Of simple design and use, it is a popular instrument but also a toy for children.
Due to the loud noise it emits, it was also used in Quebec by farmwomen to call their husbands to the field, before the mechanization of agriculture. In the Catholic liturgy before Vatican II, operated in the streets by the choir boys, it announced the offices during the Easter Triduum in lieu of bells.
It was also used as a warning that people with infectious illnesses feared in the Middle Ages, like leprosy and the plague, were passing through.
It's a bit hard to tell what's going on here; is it a mass, or an office? It surely seems more like the latter - an "announcement," via the rattle, replacing the tolling of the bells that announce the office. But then, I don't know what happened at a "mass of the presanctified" on Good Friday, either; will see what I can find out and post again if I learn something.
[EDIT: Ah. Here's something that explains things: "The hymn was also formerly sung on Good Friday when the Blessed Sacrament is taken from the repository to the altar." And that must be what's happening here; the procession is (presumably) moving the Sacrament from the Altar of Repose to the altar for distribution during Good Friday Communion of presanctified elements. I'm told that when the Sacrament is taken anywhere it's been customary to ring bells - and of course the bells are silent after the Gloria on Maundy Thursday until they are rung again at the Easter Vigil.]