What is the song that plays when Lelouch sees the memories of C.C. in episode 11? In Code Geass: Hangyaku no Lelouch episode 11, around 16:00 mark, there is a song that starts to play when Lelouch sees the memories of C.C. Again, in the same episode at 19:50, the same song plays for the second time.

I didn't recall hearing this song before episode 11, and episode 13 at 7:35 as well.

I couldn't find the soundtrack from OSTs. What is it?
It's an unreleased piece from the soundtrack called Sins and Astonishment. You can find it here:

Code Geass: Unreleased Album - 07 Sins and Astonishment