I have a intel p4 board with 5.1 analog onboard audio (digital as well). Mini to RCA routed to four amps (1-fronts, 2-rears, 3-subs, and 4-center).
From the device's control panel all six channels are configured properly. Two channel stereo is routed to all six channels properly. However, when WinDVD7 is configured for 6 channel playback I lose the .1 or sub channel. When I configure it for 4 channel I gain the sub but then lose the center.
From the control panel I can mute the six channels individually(eliminates any downmixing confusion)
PowerDVD, BlazeDVD, RioDVD all do the same thing.
Reloading drivers and os did not help.