What is the background music played in ep. 2 when we are told of the conquest of Japan? What is the background music played in the first 30 seconds of episode two, when a narrator speaks of the conquest of Japan? This is the section before the OP is played.
I have heard all of the OST tracks from Code Geass, and the closest match is in  Code Geass OST #2 - Pessimistic Time. If you listen carefully after 0:40, the main instrument's melody is the same. The style and backing are different, but it's normal for there to be some variation between the original OST and the version used in the series itself.

If you want the full list of all songs that played in the series, you can check them here.