I have a evga 780i mother bord and an onkyo ht-s800 7.1 channel system. I've tried digital audio and 4 y-adapter audio cables.
The only 2 ways you can get 'true' 5.1 - 7.1 etc is either via digital coax or a fibre optic cable. Any other connections such as phonos etc is simulated sound only. Hope this helps, KMAC.
@Decay 20 That board has fibre optic and coax out, run either one to your onkyo and you got your surround sound. You may have to change the settings in the audio software also.