Right-click on the speaker icon on the taskbar.
Choose "Open Volume Control".
Click on the "Options" menu item.
Click the "Recording" radio button under "Adjust volume for".
Ensure the "Stereo Mix" checkbox is checked under "Show the following volume controls".
Ensure that the "Select" checkbox for the "Stereo Mix" slider is checked.
Adjust the volume to an appropriate level. Note that often very low settings (as shown above or lower) are necessary to avoid distortion. If you experience loud and distorted audio when previewing or playing back your DVD recording, adjust the volume downward on this slider. Some audio drivers work well with this slider all the way down, some work well with a slightly higher volume. Please experiment to find the best setting for your system.