play GarageBand's sound instead of MIDI connected digitnal piano's sound
I want to use my digital piano (Yamaha P225) to play the sounds of trumpet. So i connected digital piano via cable to my ipad, and chose 'trumpet' in the GarageBand as the sound. However, when I play on piano, it output BOTH the piano sound AND the trumpet sound. How can i set up so that it only output trumpet sound?