That's because both original Pacha and Lightning are voiced by Owen Wilson.
Marc Shaiman originally scored the film but had his work rejected after a test screening and was replaced by John Debney. Shaiman's work can be heard in The Sweatbox (the making-of documentary on the film and its Troubled Production). He describes his take as having too much Mickey Mousing.
The movie as made was supposed to end with Kuzco building his mansion on a hill further away from Pancha's village and inviting Pancha and his family to stay in it, before Sting pointed out that if Kuzco still build his mansion after all his experiences he wouldn't really have learned anything.