Hi Ash,
I don't think VirtualDub or VLC have any way to directly communicate with the card. So you may need to set Windows' default audio device to the DeckLink card. This is technically possible and it seems to work OK for me using the DeckLink HD Extreme 3D, I'm afraid I'm not sure which card you have. To do this you would need to go to the Sound section in Windows' Control Panel and select the Blackmagic Audio option, and there should be a button, or context menu to set this to the default.
However the way the card works is somewhat different to a typical sound card. It's not strictly intended to be used in this way. Any video application that has direct access to the card, such as Media Express, will take over the card's functions only for itself, meaning you won't be able to use something like Media Express and VLC at the same time. The audio would only be output from Media Express in such an instance. Audio capture without a video signal however is not possible.