Here is a musical context for exordium:
In Dressler’s Praecepta musicae poetica (1563), compositional structure adopted the formal division of an oration into exordium, medium and finis.
Find the rest of the article here:
http://www.misson-si.net/Grove - Rhetoric and music 01.doc
Possibly I should have added to the list of categories: music/musicology/theory and analysis/opera/contemporary music/literature/novel/satire
Do you have an alternate suggestion to exordium?? Overture perhaps?
As to Mozart and the maracas, see "contemporary music" in categories - and the fact that this is a work of fiction.