Asus Xonar DGX User Manual page 26

Q7: Why can I hear and record sound from other recording devices when
Wave/Digital is selected in Record on Vista?
Answer:
Different from Windows XP, Vista has a multiple devices/streams concept in audio
structure, in which the concept can be categorized into Digital Input and Analog
Input. In other words, multiple recording devices can be used and working at the
same time. In this case, Wave In or SPDIF-In, considered as a Digital Input, is
an independent recording device from Analog Devices such as Mic In, Line In,
or Aux In. Therefore, if you have analog inputs other than Wave In and set to be
monitored, you are able to hear and even record sound from those analog devices
even Wave In / SPDIF-In is selected as the default recording device.
If you don't want to hear the analog input sounds at that situation, simply turn off
"monitoring" function on other analog input devices.
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