I use my MOTU specifically for Sonar and my internal Realtek for Windows Media Player, VLC, web audio, etc.
To use ARC with the Realtek, I use VAC (Virtual Audio Cable) to route audio to Pedalboard2 and from Pedalboard2 to the Realtek. Pedalboard2 hosts VST's and that where I load ARC which stays active at all times for non-Sonar audio.
I've been doing this for quite a while now and it doesn't interfere with Sonar or the MOTU.

Hope this helps somebody...

Dude, THANK YOU!!!! I've wanted to do this FOREVER. I hate listening to my ITunes stuff or Pandora without Arc.
 
I can't wait to try this over the weekend.
 
Thanks so much for taking the time to post this, I really do appreciate it! :)

This is awesome.  I also use a realtek for random browsing and stuff, but I use Winamp (which supports VSTs) as a music player and it routes to my studio monitors through ARC.  This way whenever I am listening to music, it is through my monitors and through ARC, so I really get a feel for how all my "real" music sounds though my ARC'ed monitors.  
 
Winamp supports VSTs via a method I learned from this forum a while back.  The newer version of Winamp has a web browser built in (although I have never used it). I suppose if you used the browser in Winamp, it would play any web based music through the ARC and your monitors as well.  Nice. 

I mostly use Winamp as well for the same reason but often use Pandora and a few other audio programs. Looking forward to having Arc for them all!

can a program like 'WINAMP' run thru pedalboard?
 
that's what i currently use to play wav and mp3's on my recording system......
 

Winamp can actually use VSTs, so you can use Arc in it. (at least the version I have does)

Pedalboard acts as the vst host and VAC routes all audio designated to your chosen soundcard to pedalboard and out of pedalboard to the soundcard.
This is a way to insert ARC into the signal chain of the chosen soundcard so WinAmp, web audio, iTunes, VLC, WMP, etc. etc. etc. plays through ARC.

Example:
before: web page (or WinAmp, or iTunes, etc) -> internal sound card -> speakers
after: web page (or WinAmp, or iTunes, etc) -> VAC -> Pedalboard (host for ARC, or any other VST) -> VAC -> internal sound card-> speakers

Just out of interest......  I use a motu ultralite.... how does sonar performance behave having two audio drivers active?

If you look at my sig, you'll see that I also use an ultralite, no problems whatsoever!