i have the exact same problem. My surround is the latest LG and cost 1000 bucks cannot get other than 2 channel. Computer cost me 2000 bucks.
Nothing seems to help, i also bought optical cable.. didnt work either.
Installed drivers over and over again, nothing helped. Gave computer to technical support in a private company, 50 bucks later. No result
i have HD 7970! IF you somehow make it works please forward your advice to my email
email@example.com and i will reply here if i get it to work