Please help, I can't connect subwoofer/center speaker.
When I connect those speakers, jack sense application doesn't sense them and I don't have sound on those speakers.
Other speakers work fine.

the software of most sound devices has an option to disable the auto detect feature. If you don't find it, maybe you could tell us which sound card you're using.

It has that option, but even if I manualy select proper speakers still I have no sound. I'm confused because other speakers (rear and front) work just fine.
Sound card is onboard audio device realtek ac 97 and it has three jacks, green, blue and red (out, in and microphone), but when I turn on 6 channel mode, microphone and line in jack are used as subwoofer/center speaker and rear speaker output.

1) Update you audio driver and tell me what happens.
2) If you happen to have a Logitech webcam, uninstall its software and driver and if the problem is solved install only the driver.

Than You
I tried all that (don't have Logitech, but Genius webcam and I uninstalled it)
but still no sound from sub/center speaker. New driver doesnt help.
Is there any way to bypass sound config and to force onboard card to send sound trough center/sub channel? Probably connector sensing doesn't work.

Well man, I'm out of options. You should try calling gigabyte, if no luck return it or buy a cheap audigy.
Ahhh, and make sure its not your cable or speakers. plug some headphones and listen for center channel audio (one side)