different songs for different countries is a must!
I am learning to sing them, because I am weirdo.
For the French: Les Marseilles of course & the 1812 overture (which also works for the Soviets)
In fact, national anthems across the board. they're good music, most of them. Many are offensive to people nowadays, be careful who you sing them too!
I already know Rule, Britannia! as well. The British Grenadier's march and Scotland the Brave also good choices for the UK.
for the Germans, Ride of the Valkyries as well, unless you played too much HOI3 and can't take it anymore.
For any country, "Mars, Bringer of War" by Gustav Holst as well as his military marches are good too. ok I need to stop here or I'll just be naming songs all day.
I am REALLY looking forward to the music that comes with the game; paradox does good music and I think HOI4 is going to sound good. the score has sounded really good in the WWW videos. would love for them to post a track on the forum or something as a teaser.