Ahm ... Sorry ...
Main question, boiled down to an understandable level:
If you play a Theora and detect the Audio Codec A52, which happens to be the codec used to pump Dolby Digital or DTS through the digital connectors of the computer, then just pipe the stream it to the soundcard without altering it or changing it in any form. This enables soundcards to decode the stream or handle it to external decoders, the effect beeing multichannel audio in theora files.
VLC Media Player had to use a special audio driver for this, for it's normal driver alters the sound stream. This special driver is called "waveout extended" and I have no clue what exactly it does. I think it's specially programmed to do nothing.
The only Problem on the WME side of this playback problem is: there might be more than one Audio Stream, and they all must be passed through.
Wether or not the player wants to use Digital Surround Audio is not your problem, this is up to the designers to provide proper theoras for digital surround and stereo waveout. I'm just asking for the piping here.