1. Open up Master Volume's window by double click on volume icon in taskbar.
2. Choose Options>Properties which will open up Properties' window.
3. In Mixer device list, choose your input device. E.g " Realtek HD Audio Input". Then click OK.
4. Tick the checkbox in Stereo Mix's column to use it. Adjust the volume as desire. :)
1. Go to Control Panel>Sounds And Audio Devices and navigate to Audio's tab.
2. Click on Volume's button in Sound Recording's section. And repeat step 4 like above.