GAME BOY SYSTEM INSTRUMENTS EDITOR MENU:
You can control the Game Boy sound using STD Instruments, you can set a Volume Envelope, an Arpeggio Envelope, a Duty Cycle Envelope and a WaveForm Envelope.
And in order to define the shape of the WAVE channel you have to use the WAVE Editor. You can use more than 1 WAVEFORM, and in order to switch between WAVES you could use the "Selected WAVE" display in the top part of the window, you can also change the selected wave in the instrument list, you have to use the 10xx effect to switch them while a song is playing.
Remember that the max volume value for the WAVE Channel is 3, so if an envelope value is outside this range, the result will be always 3.