My favorite background music is... well, I don't know the name of it, but it's used during that one split-screen sequence where the left side focuses on Tsukasa and the right side focuses on Kagami, and there's a clock in the middle. It's also used as the OP for the OVA, as well as the Extras menu for volume 3. It's also in other places, but those are the best in terms of hearing the whole thing.