The case is I'm using kernel steaming for foobar2000 with one of my soudncard's digital output. The XP default audio device is still my onboard crappy coundcard. I also configured some player like WMP to use the new digital output. But I can't make it with POWER DVD and VLC. Anyone know how to do it?

You can try to enable digital output in VLC by navigating to Settings -> Preferences -> Audio and enabling "Use the S/PDIF audio output when available" checkbox.

PowerDVD can be configured by selecting Preferences -> Audio tab -> Speaker Environment -> Use SPDIF. Additional support is available at Cyberlink Support Site.