In the Quad Capture's manual it says the following. Try this:
Monitoring Function Settings (Windows 7)
If the monitoring function of Windows is enabled, the input sound may be doubled (each sound is
echoed) or feedback may be heard.
Use the following procedure to disable the monitoring function of Windows.
1. Open the “Control Panel,” click [Hardware and Sound], and then click [Sound].
If you have selected Icon view, click [Sound].
2. In the “Recording” area, click the QUAD-CAPTURE’s [1-2 (QUAD-CAPTURE)], and then
click [Properties].
3. Click the “Listen” tab and clear the [Listen to this device] check box.
4. Click [OK] to close “Properties.”
5. Click [OK] to close “Sound.”