Who says that all music of D:OS2 must be serious music.
If done well, also folk or pop music may fit to a game.
D:OS2 has a more or less middle age setting. So most modern pop music would not fit.
But maybe some fictional folk music that is based on the culture of rivallon might fit in some places.
Fictional folk music means that it reminds the player on some real world folk music, but it does not need to care for the rules of a certain type of folk music and the text (if existent) refers to the culture or history of rivallon.
I still remember the In Extremo stage event in Gothic 1.
You can't compose folk music, though, that's why it's a separate category. If you compose "folk" music it's either u-musik or serious music, depending on the architectonic, development, theoretical and logical justification of the music. Folk music has no composer and isn't composed, it's passed down from tradition and changes along the way. Its history cannot be traced in any way (tradition is the opposite of historicity in history studies) and is just there. Though composing folk-like music for the game isn't a bad idea. It will confer a feeling of tradition in the game where it's appropriate.
About it being serious music - it all depends on what the intent and skill of the composer is. They can make it serious music or not, Arcanum's soundtrack IS serious music btw and it works great -
https://www.youtube.com/watch?v=5LHLAcd14b4 
