Unable to adjust sound levels in Options\Settings\Sound

I am flying an OpenSky Freeware plane that references the 737-800 sound files and the engine sounds are very loud. I have tried adjusting the engine, cockpit and environment sound levels down and it has no affect. I have even tried unchecking the boxes for the engine, cockpit and environment and still have engine sounds blasting at me. If these sounds can not be controlled by the FSX Options sliders how can they be adjusted and how are they controlled??Thanks for any help on this one. Randy