Derek is correct. If You have a realtek card (I don't know about support from other manufacturers) You set it to stereo mix, and set the stereo mix to default microphone. You can then create a source node linked with your mic using navigator.getUserMedia
.. Google and You Will find it.
I just made a fiddle for you: Audio from soundcard
EDIT: the link now goes to the right version of the jsfiddle (the other one had the bars wrong).
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