The audio attenuator HI/LO switch is used to adjust the output volume for different audio sources. It has no effect on noise reduction.
Before using your headphones for audio, note the setting on the switch. The switch is at the headphone end of the audio cable, which is inserted into the headphone. If it is in place in the headphone, take hold of the audio cable as close to the left earcup as possible. Gently pull it out to access the switch, as shown below.