You recording device might be disabled. Right click on the speakers icon locate it on the bottom right corner, go to "Recording Devices", right-click in that window and select "Show Disabled Devices". This should bring up the "Stereo Mix" option. Then right click the Stereo Mix option and select "Enable". Now you can select it as the default recording device and any program you use to record sound should now directly record the sounds playing on your PC. If the issue continues, please open a support ticket at:
https://www.nch.com.au/support/supportcontact.html?software=MixPad&support