So far the music I've heard in the Civ6 videos is great,
America's theme is some folksy violin music, don't recognize the song
Aztecs' has Native flute music
Brazil has the Portuguese sounding guitar music
China has the guzheng, with some people saying the song is Mo Li Hua
Japan has the flute, and it seems to be Rokudan no Shirabe again
I saw England in some videos, but didn't notice their theme music, I also didn't notice any war themes (please let there be war themes!
)
The other music is great, I recognized Epitaph of Seikilos (Greece's theme in Civ5) and a famous Medieval (I think it's Medieval?) English song that I can't name