Just added a few extra song.txt documents so you can have more options on when/what you want to cause the music to change.
Currently 4 are there
1900 is the Default to change because its a simple choice
1901 is another because it is technically the first year of the 20th century, Queen Victoria also dies January 22nd 1901, ending the Victorian Era, so its a good choice to shift the tone of the music.
Then there's options for the discovery of either great wars or world wars to change the music, so it will be more dynamic and dependent on what's actually happening in your current game.
All the music is the same as 1.0