Q:  I want a sound card that will accept the Dolby Digital 5.1 audio signal 
    from an external source (in this case, an Xbox 360), decode it, and output
    it using the computer's own 7.1 speakers.

A:  There might be a misconception here. The Dolby Digital decoding done in
    a computer is usually only available when playing a DVD movie disc, using
    a DVD player software such as WinDVD, PowerDVD or in WinXP Media Center
    Edition, Windows Media Player 11 with a suitable Dolby Digital plugin. 

    These programs will not accept the incoming digital signal from the Xbox
    digital output, or any other external playback device for that matter. 
    This is pursuant to licensing agreements between computer manufacturers,
    Dolby Labs, movie studios and copyright holders. 

    Our soundcards do not incorporate Dolby Digital decoders and we do not 
    know of any soundcard that incorporates a Dolby Digital decoder that will
    accept incoming AC3 data streams. It is expected that the only devices
    commonly equipped with Dolby Digital decoders will be surround sound 
    speaker systems and home theater (A/V) receivers. 
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