There are actually some pretty good reasons not to use a soundcard as a DAC. One of them is that it's actually really easy and cheap to build something like the Pangolin FB3 (which actually has < $100 of parts on it) these days. I daresay one could get all the functionality of that with just a single AVR processor, one of the FTDI USB chips, some SPI DACs (about $1 a pop) and a few connectors... and in fact, I have such a device on my bench right now. PCB layouts and schematics will follow when I've resurrected my poor abused scanners (software testing is bad for scanners ) and fixed it all up. I haven't implemented DMX512 yet because, personally, I think DMX512 control is going to go the way of the dinosaur; USB all the way, yo.
On another note, seriously, and no kidding: James, you HAVE to put a decent GUI on that thing. It's by far the most capable open source laser software. Please, please, please, put a WIMP interface on it. Please. That should be your main focus now. Not getting your name put on things you didn't design.