Bueno, consultando en el foro hermano mundodvd.com y con la ayuda de San Google he encontrado los siguientes enlaces, que pueden resultar de interés:

1º.- Hilos donde se trata el tema de activar la salida SPDIF:

BUSCAR ACTIVAR SPDIF

2º.- Hilo chicheteado donde se explica todo. En principio sobre Zoom Player:

USANDO ZOOM PLAYER

3º.- Hilo sacado del foro de la web de Creative:

FORO CREATIVE

De aquí copio lo siguiente:

"Movies wth Dolby Digital (AC3) & DTS
Movies on DVD often use encoded audio in the form of Dolby Digital (AC3) or DTS. *This means that all 6 channels are compressed into a single stream of digital data. *A decoder is required to extract the channels and send the sound to the correct speakers. *There are three ways to do this.


1) Pass the AC3/DTS signal to a external decoder.

This is where your optical/coax connection is used. *To go down this route, you must first have a set of speakers or a decoder that supports Dolby Digital. *In your DVD software, select 'SPDIF' as the audio output method and 'SPDIF Pass-through' in your soundcard options. *What this does is tells both the software and soundcard to leave the audio from the DVD in compressed form and send it straight through the digital outputs. *Your digital speakers/receiver will do all the decoding to give you full surround sound. *Please note that this is the only way to get 6 channels through an optical/coax cable.

Lo que no me queda claro es si tengo que hacer algo en la configuración de la tarjeta; por ninguna parte veo lo de 'SPDIF Pass-through'. Lo único que veo es usar sólo la salida digital de audio SPDIF, pero esto no es necesario ya que esta salida funciona siempre (bien que remezclando a estéreo).

2) Decode the AC3/DTS with your Sound Blaster.

You should choose this option if you don't have an external decoder. *In the DVD software, select 'SPDIF' as the audio output method and make sure that 'SPDIF Pass-through' in the soundcard options is off. *The DVD software will now pass the compressed audio to your soundcard, which will then decode the data. *The soundcard will output the 6 channels using the analogue connections (or digital DIN, more on this later). *If you try to use a digital optical/coax connection to your speakers, you will only hear faint sounds from the front left/right speakers.


3) Decode the AC3/DTS with software.

If you have a soundcard that doesn't support Dolby Digital (like the SB Live! 24-bit) and you don't have digital speakers, the only way for you to get surround sound in movies is to use software to decode the AC3/DTS. *The free versions of DVD software you get with hardware often don't support this feature, *you will most likely need a full version. *First select 'Dolby Digital' or 'DTS' decoding in the DVD software audio options (you must select the right sound format for the movie you are watching). *Then make sure that 'SPDIF Pass-through' in the soundcard options is off. *As with method (2) you must connect your speakers with the 3 analogue cables, not optical/coax."

En definitiva, los problemas que quiero solucionar son los siguientes:

a) Reproducir DVDS respetando el tren de datos 5.1. Parece que solucionado con Power DVD. Falta probar.

b) Reproducir ficheros codificados en DTSWAV. Con el VLC player se consigue, pero sólo consigo obtener la mezcla estéreo PCM a 24/96.

c) Reproducir otros ficheros del disco duro codificados en DD o DTS, con independencia del codec de video o de audio utilizado. De momento he cambiado las opciones del Windows Media 11, activando la reproducción por SPDIF eliminando la opción de reducir a mezcla estéreo estéreo PCM 24/96. Probar. Si no funciona, probar con el Zoom Player más filtros, pero esto ya es para nota.

De paso voy a ver si actualizo los drivers, etc. de la Audigy.

Seguiremos informando.