Ì stumbled upon this sketch. I love the part the orchestra is about to play at 4:40. Is this something they came up with for this performance or is it a well known piece of music I don't know about? I'm not familiar with classical music.
It's Beethoven's Egmont Overture. It's a pretty well-known piece among classical musicians, but the layperson likely won't recognize it. (But note that this was filmed in Germany in 1978, so a lot of the audience may have recognized it.)
It's a great piece. I linked you to the same part (the ending), but definitely check out the whole thing!
Edit: For completion's sake, earlier in the video they play Beethoven's Coriolan Overture.