When musicians work with additive synthesis, they will often wrap envelopes around the sounds, and even around each added sine wave. An envelope changes the amplitude over time—it might start out small, then grow (rapidly or slowly), then hold at a certain value during the sound, and then drop before the sound ends. That kind of pattern is sometimes called the attack-sustain-decay (ASD) envelope. Pianos tend to attack quickly, then decay quickly. Flutes tend to attack slowly and sustain as long as you want. Try implementing that for the sine and square wave generators.