On the side of the organ are the stops of the registers. When you move over them with your mouse, you'll see what registers they are and when you click on one of the stops, you'll hear the sound of the corresponding register.
The registers are described below.
Register 1 are concertflutes.
Register 2 are bourdonflutes.
Register 3 are the violins.
Register 4 are the clarinets.
Register 5 contains the wooden piccolos.
Register 6 are the so called quinte (metal flutes).