well... I already used g751jy with 5.1 system (from creative), connected with a cable that had 3 stereo 3.5 jacks, same colours as yours.
just make sure to set up for 5.1 the realtek driver... it should pop up after connecting the jacks
it worked g8t
- if sth goes wrong, check in system tray, right click on sound icon and choose "sound playing device" (sth like that, i don't have EN win ver...)
Then select Realtek High Def Audio and click Configure... there you'll find all options to set it up
good luck!