ariel, I'm not sure I understand exactly what you're saying, but yes, the source will need to have an AC3 stream to be effective in the Dolby Digital mode. I think the soundcard configuration would be the same regardless of whether you play a DVD, or other AC3 enabled source, that is, in Creative Control Centre, disable the "decode AC3 and Dolby Digital" option.

For instance if you have an .AVI with an AC3 stream, whatever player you are using may need an AC3 plug-in, and should be set to AC3 passthrough mode. However a topic here:

http://www.dvdrhelp.com/forum/viewtopic.php?t=184380

has someone with the same problem, and suggestion is maybe try different players.