To record the “Stereo Mix” or “Wave Mix” on Vista with Audio Record Expert, you will need to enable the device first.

1. Select sound from the control panel.
2. Select the recording tab.
3. Right click on the background of the tab and choose “show disabled devices.”
4. Right click on Wave Out Mix and click enable.
5. Now it should work the same way as Wave Out Mix in Windows XP, allowing you to record any sound your computer makes.

Read more on

http://www.downloadsquad.com/2007/01/15/how-to-enable-wave-out-recording-in-vista/