At Celadon City, enter the Celadon Condominiums next to the Pokemon Center. Use the elevator to reach the third floor. When you exit, your friend will tell you about the GB Sounds. Talk to the man next to the globe after collecting all of the Kanto badges to get the GB Sounds Device. If you turn on the GB Sounds option before entering the Union room, then trade with someone, it will play the classic trade music.