@KaoruTakarai88: As tech_lover said, this is a classical piece by Gustav Holst called "Jupiter". It is from his Planets suite, and is actually a very popular classical song (along with Holst's "Mars", also from The Planets).
@KaoruTakarai88: Well after researching about it, I found that it is actually more complex than that. The very original music piece is called "Jupiter" from "Gustav Holst"'s orchestral suite "The Planets". He adapted it a bit and called it "Thaxted" to be accompanying an old British patriotic song called "I Vow to Thee, My Country". Each of Sarah and Ayaka is singing her own lyrics to it.