Volume (VOL)
WARNING: This page is about a different car, the 2002 Pontiac Montana, 2002 Oldsmobile Silhouette, and 2002 Chevrolet Venture. However, it is still accessible from the selected car via links, so may be relevant.
Rotate the volume control in order to increase or decrease the volume. The top VOL knob controls the top headphone and the bottom VOL knob controls the bottom headphone.