I feel like the soundtrack and diplomacy music could be improved in several cases.
The most obvious instance to me is Macdonald and Trudeau, which for some reason use the Viking diplomusic. Not only is it un-Canadian, it is also rather unpleasant to the ear. (Isn't it?) I suggest replacing it with the Canadian national anthem, of which there is a rather good (and public domain) version
here.
Speaking of the Viking diplomusic, I think it is also inappropriate for later Scandinavian leaders (Gustav Vasa and Gerhardsen). I suggest replacing it with the praeludium from Grieg's
Holberg Suite (first 2:30 or 2:40 of the complete suite, depending on the recording; ignore the recorder ensemble version, it's rather weird).
There are a couple more things I would change in the diplomacy music, but I want to move on to my main suggestion: Add a new soundtrack for the Global Era. As of now, both the Global and the Digital Eras use the same original Civ IV soundtrack made of John Adams' minimalist music (which I love, by the way). However, this music has a futuristic feel to it that doesn't quite fit the early 20th century; moreover, Adams composed it mostly in the 1970s-80s, which would correspond to the early Digital Era. I also think that changes in soundtrack are the most salient way in which the game indicates passage to a new age (provided you play with sound on), and considering that the Global and Digital era lack aesthetic differences in general, music would be a good distinguishing feature.
I propose that we create a new soundtrack using music by post-Romantic classical composers from the early 20th century. One issue is copyright status, as some of the composers that would be worthy of inclusion are off-limits due to dying after 1947. But that leaves quite a few who did die early enough. I'm thinking Debussy, Ravel, Gershwin, Respighi, Satie, Bartók, Rachmaninoff, Holst.
The best way to do this would probably be to update the optional soundtrack module. (The diplomusic suggestions above should probably be in the base mod, though.)
I am willing to do research into finding appropriate tracks, and I can start a thread to collect and discuss suggestions. Eventually I can prepare a pull request (though I will most likely need a bit of technical help regarding which files to edit, etc.). However, I first need to know if Leoreth thinks it's a good idea. And if so, what are your criteria for the inclusion of a track, in terms of copyright status (is creative commons ok?), length, and aesthetic qualities?