This is the way you used to record through your soundcard with Audition 3.0. I haven't tried it in awhile, I seem to recall a Windows 10 update broke the functionality.
Occasionally a Windows update will change settings so that Adobe Audition will not record from the PC sound card. A message appears telling you to change settings in "Hardware Setup".
1) Open Windows Control Panel > Sound > Manage Audio Devices
Click the Recording TAB and enable the Stereo Mix device
2) In Audition, open Hardware Setup.
Click the Multitrack View TAB
Change the Output dropdown choice to Input
Windows 10 – in Multitrack View in AUDITION, click the control panel button to open up the window where you can enable the soundcard.
Select the soundcard to enable it in Audition.
You should now be able to record audio playing through the PC souncard.