The USB device is equivalent to a sound card, it doesn't need an exiting sound card to work with or through or anything like that.
One caveat is that while most consumer sound cards have a MIDI synth chip on them, none of the USB device that I know of do. So they won't be able to replace that part of a typical consumer soundcard's functionality.
The emacs have on-board sound, though, don't they? I suspect the USB device can be used in addition to the normal onboard sound without any problems.