Hi

To record / capture computer sound you need to set the audio source as Stereo Mixer.

For Realtek HDA audio card / chipset this will probably be disabled and you need to enable it as follows:

Windows 7

1. Open Control Panel, Sounds
    
2. Select Recording tab
    
3. Right click in the centre panel on a blank area
    
4. Select Show disabled devices and Enable disabled devices
    
5. The stereo mixer should now appear as shown in the image below..

For Windows 8 

Open Control Panel, Hardware and Sound, Manage audio devices

then follow steps 2 - 4 above

For other Audio devices you may need to search the Internet for instructions on how to turn on the stereo mixer or similar device.

NOTE: to avoid recording the system sounds you should turn the appropriate volume control to zero or disable system sounds.

HTH

John