No sound volume Change with canopy open or closed

In A-10C sitting on runway world sound set to 25 opening or closing canopy there is no change in engine volume. Both hear like helmet on and off is effected.

I want very quiet engines with the canopy closed to mimic RL

ok, it seems in cockpit controls external engine sound in the cockpit, seems counter-intuitive. You'd think in cockpit would control in cockpit generated sounds, not the external engine sound.