Hey Unseen, check it out (Audio over HDMI ):
https://github.com/charcole/NeoGeoHDMI

Hey Psycho,
I think digital video output is compliant with ITU 656 and here you have more information:
http://gamesx.com/wiki/doku.php?id=av:nintendodigitalav

Any chance that you project evolves to HDMI(digital video + digital audio) ?
Spartan 6 suppports LVDS outputs