If you have an older computer, you could just build one of those parallel/serial cables for 5$ or so.
Internal "prosound" mod - I've seen some schematics floating around for that, but I don't really think it's necessary. Those microjacks fit very tightly (much better than most 1/8in jacks), and the output is loud as f*ck anyway.
For those interested in demos for the platform, check the list I compiled here: http://www.omnimaga.org/index.php?topic=16687.0