Since the Ac3 support is a little bit complicated :( I decide to trancode all the 5.1 Ac3 to 6 ch Ogg Vorbis (with automatic max gain). So... I hope people can tell me more about your experience ...detail with your machine for this process. How fast is the speed ? 2x, 3x ? Thank you.

@DSPguru, Thank you for your FAST reply ;) How if I want to mux it with the AVI with Koepi's OggMux, so I prefer use Ogg than Aac. I think Tobias's OggDS filter can handle the 6ch Vorbis, isn't it ? I also notice that the latest Vorbis Winamp plugin in_vorbis v1.2 now can play the multi-channel Ogg vorbis. Any ideas ?

yeap, multichannel vorbis is supported by both OggDS & Winamp :).

@tiki4, Yep, I have the same problem like yours with the same card and speakers ;) The 5.1 ogg not played well in my machine since the center sound is moved to the front right speaker :( ...but at least the AC3 playback is OK only with SPDIF, still have problem with analog 6 channels, center sound is moved to the front right :( ... using WinDVD 4.0 iviaudio.ax. Thank you.