I use my audio interface for music apps, like Sonar and any stand-alone synths (Dim Pro, Rapture, Kontakt).
I use the on-board sound chip for the default Windows audio device. I route the output from the speaker jack on the back panel to a couple of inputs on my audio interface, which allows me to have Sonar and perhaps a YouTube clip playing at the same time. If I need those 2 inputs from the audio interface, then I change the default Windows audio device to be the HDMI connection to the TV I use as a display for the computer, and it routes sound to the TV speakers.
In either case, I do not have conflict running in the above manner.
Bob Bone