I have just built a PC from scratch all with brand new components and parts. I have a Gigabyte GA EP35 DS4 motherboard with on-board sound. I installed the drivers for HD audio which were on the CD and configured it to play sound from the optical output. My surround sound system is a bit dated but still working fine. I'm using a Creative Inspire 5.1 Digital 5700 setup and never had any problems at all with it.
The sound only seems to coming out of the subwoofer (the software thinks it's one of the front speakers) and one of the front speakers. There's no sound coming through the rear or centre speakers.