Love. Music. Freedom. These are the universal themes at the heart of “We Live in Cairo,” a new musical by Daniel and Patrick Lazour, which is having its world premiere at the American Repertory Theater. Set during the January 25 Revolution, the 2011 uprising in Egypt, the work, under the music direction of Madeline Smith and music supervision of Michael Starobin, celebrates the hope and exuberance of the uprising, even as it acknowledges the turmoil that has followed.