I’ve been using Sonar Artist and a Focusrite Scarlett 6i6 (1st gen) on Win10 for quite a while.  I’ve often found it useful to play along in Sonar with audio from Youtube, iTunes, or Groove music.  Sometimes I’m trying to play with ideas for scoring over something, or just trying to learn a piece.  It’s always worked fine.
 
Now I bought a new PC and reinstalled everything, moved the USB 3.0 cable for the Scarlett from the old PC to the new, and now Sonar gives a whole series of Audio Driver Error if other sound is playing. “The following drivers either do not support the current audio format or are In use by another application”: Focusrite USB 2.0 audio SPDIFL, DAW 7. Then a “missing audio outputs” dialog saying “the following audio outputs not available” missing device spdif. If I pick another device, it just says that doesn’t support the format either.
 
The versions of Windows, Sonar, and Scarlett drivers are all the same.
 
Preferences: same audio playback and recording settings as on the other PC that worked (ASIO, no dithering, no sharing drivers w other programs).  I tried sharing drivers too and still can’t play simultaneous audio.
 
Any other audio programs can play over each other, like iTunes and Youtube at the same time.  Sonar is the only program that has a problem and it’s with any other application.  If I do Sonar first the sound is fine, and then any other application won’t play sound (how it reacts depends on the application – anywhere from silence to hanging).
 
I’ve gone through everything I can to make sure it’s the same and yet it still doesn’t work. I even uninstalled and reinstalled the Scarlett drivers and mix control.  Please help!

Are Windows and SONAR set to the same sample rate?

RothStudios
 
Preferences: same audio playback and recording settings as on the other PC that worked (ASIO, no dithering, no sharing drivers w other programs).  I tried sharing drivers too and still can’t play simultaneous audio.
 

I am assuming the above refers to SONAR preferences. There are also exclusive mode preferences in the "Sound Card Options" in Windows (far right tab at the top when you right click and select "Properties" of a device). Check to see if both "Exclusive mode" options are unchecked there.

Sort of not an answer to your problem but a solution to how I deal with this. It is easy to grab the audio from uTubes and then import it into Sonar. This is sometimes a cool way to build a song and I have done this a few times. Sometimes the songs even follow a steady tempo if your lucky so midi quantizing works without issue. 
 
Any how I use this little on line app. It will nag you for paid upgrades but otherwise it works. For multiple downloads I will hit the  refresh page to re set it otherwise it nags.  Used it for over a year now, best thing is you don't have to install anything it runs from a web page.
Copy / paste  the U tube ULR to the box, You'll see in the "Download in Browser" option half way down the page,, that opens a download button with options for the video quality plus just MP3 audio.   http://en.savefrom.net/  
 
Disclaimer- be aware of copywrite laws as use of this app to copy music may be unlawfull in some countrys. There, I said it so I can sleep at night.. 

Scook - you got it!  Windows was set for 48k and Sonar for 44.1.  Fixing that also got rid of an annoying pop that would happen when I switched sound sources.  Thanks!