Discovery 640/645: How To Adjust the Volume
How To: Adjusting the Volume on the Discovery 640/645
There is a black volume bar on the outside of the headset. To increase the listening volume of the headset, press the upper end(+) of the volume/mute button. To decrease simply press the lower end (-). The microphone volume is set. There is no way to increase the microphone volume on the headset.