Unless your HU supports Optical connections then no, though is coaxial digital? I can't remember that one, or whatever it is. I just remember I have had several different connections going from my PC to theatre receiver, and IIRC optical and one other were both digital, but optical is the more common I think.
If you check the FAQ Emporium, and then the FAQ TO THE FAQ'S sticky at the top, there is a link in there that points you to a long thread about hooking your audio up to your carPC.
I wish I could offer more help but I know nothing about getting true 5.1 since that isn't what I am looking for =P