So, I took a look at how they did the game's music, and noticed how easy it was to edit... so I started hunting for traditional music from between 1400-1800 or so in order to create a DIY "music expansion pak".
The first thing I stumbled into was an artist called Pavle Aksentijevic, who does medieval Serbian & Byzantine psalms that are truely stunning -- a whole albumful of mp3s, plus pictures of Serbian icons. Lovely.
Of course, I shouldn't choose just Serbian music, since EU II allows you to play practically any country in the world. So here's an idea -- feel free to point out websites which feature free, non-RIAA mp3s of traditional music from the 1400s-1800s. Nationality doesn't matter -- quality does. I'll also continue to update this post to point out other sites I find.
When I'm done, I will create a nicely compiled post of favorite mp3s, sites, etc. Think of it as a way to get a whole series of free "best of" compilations. KTEL's greatest hits of the 1400s, 1500s, etc.