In XP, there is
Control Panel -> Sounds and Audio Devices -> Audio
Under the Sound Recording device selection, there are 2 buttons:
The problem is that with Realtek Audio Manager, the button "Advanced..." is frozen... (label in light gray, impossible to click)
And I just realised that the VLC playback is 2 seconds late, which pretty much spoils the whole thing.
Anyway, thanks for trying.
ps: I forgot to mention that this option is also not available in the Realtek Audio Manager (which seems to be just a clumsy copy of windows interface with "dolby surround" written on every form)