Rebecca Valette (Nathalie's mother) sent to me the following email:
I noticed that at the soirée of the Marquise de Saint-Euverte, Swann listens to this flute solo:
Thank you, Rebecca! That is gorgeous. (In the Modern Library translation, for those who would like to look up the reference, the Narrator's mention of the "air from Orfeo" is on p. 466.)
Inspired by Rebecca's research, I couldn't help myself and had to look up the "pianoforte intermezzo (Liszt's "Saint Francis preaching to the birds") which had succeeded the flute..." (ML, 466).
Here it is:
After the Liszt, the pianist plays a Chopin Prelude and a Polonaise, which are not specified.
And then, following a break, another set of music begins with the Vinteuil sonata... which is, as we know, thought to be based on Saint-Saëns' violin sonata No. 1 Op. 75. Here is a performance I found which shows the musicians as they play: