For whatever the worth, I have a Presonus AudioBox 1818VSL audio interface, BUT I do not have the Windows default audio device set to use it. You may be able to have it set to your interface, but I have had conflicts in the past doing that, so instead have opted to have only music applications such as Sonar actually point to the ASIO drivers of the interface.
When I DO want to record something like a song from YouTube, what I do is to use one of a number of freeware video download programs that are out there (I happen to use YTD Video Downloader), and it lets me download videos from YouTube and save them on my computer.
Then, also contained in the YTD Video Downloader software, is functionality to convert the downloaded video clip into wave or MP3 format, which I do.
Then, I am able to simply import the MP3 or wave file into an audio track in Sonar, and then I can do whatever I want in Sonar with that imported audio.
It isn't the same approach that you seek, but it DOES get the job done, and does so in better audio quality than the approach of sending the song through the interface with a cable loop.
By the way, I use an HDTV as my display monitor, and connect to it from my computer with an HDMI cable. THIS is what I have my Windows default audio device set to, since the HDTV has built-in speakers.
Bob Bone