Volume (VOL)
WARNING: This page is about a different car, the 2003 Oldsmobile Bravada, 2003 GMC Envoy XL, 2003 GMC Envoy, and 2003 Chevrolet TrailBlazer. However, it is still accessible from the selected car via links, so may be relevant.
Rotate the volume knob in order to increase or decrease the volume. The left volume knob controls the left headphone and the right volume knob controls the right headphone.