I've read around a few topics on this, and tried a few things, but it hasn't helped.
The basic situation is I've just bought an X-Fi Platinum, with a view to using the optical out on the front I/O panel to playback DVDs with the originally intended sound. It's proving far easier to say it than to do it though.
I've installed the AC3Filter (v.), and set it "Use SPDIF", and set Output to "As Is". Under the system tab, I've set to use AC3 Filter for all the different audio types, set SPDIF passthrough for all types, and for SPDIF options I've turned off "use AC3 encoder". I'm not sure whether this is all set correctly.
In WMP when I try to play a DVD I see a message about it trying to get a codec, then it fails to play. Note that it used to work, but just in stereo, so something's changed, albeit completely broken it.
In PowerDVD I don't have any option for choosing SPDIF.
In VLC I've chosen to use SPDIF if available, but just get stereo output.
(The DVD I'm testing with is DTS)