It seemed to play any piece found in that folder or its sub folders, which includes all eras.
The music at the very beginning is very basic, almost sounding like background sound really. I didn't recognize it as music at first. I think it just ramps up over time, appropriately enough.
The music toggle is CTRL-M. Pressing that a few times and listening to the difference should tell you whether music is on or off for you (assuming you haven't specified a custom folder yet).
You can also click the "No Sounds" checkboxes at the bottom of the various sliders on the Audio Options screen, and listen for the differences to tell you whether anything you're hearing is actually classified as music or not.
--Julian