It's been a while since I tested out the onboard sound on my MB, so I may be off base, but here are a few ideas.
There is a program you install (I think it's with the drivers) that you can set up different configuration - it may be the AVRack you mention.
Anyway, in this program when you change the config (Mono, Stereo, 2.1, 5.1, etc) you need to look at the outputs in the display. Depending on the configuration you choose, the outputs will go to different jacks. It should show on the display of this program what goes to each color coded jack.
Hopefully it's as simple as having the output on a different jack based on the setup you've selected.
There may be something in the BIOS for this also, I just don't remember as I didn't stick with the onboard sound and it's been some time since I set things up.