Can't Get Onboard Sound To Work Properly

I have my speakers hooked up to the motherboard itself (onboard) as my sound card is not compatible with Windows Vista 64 Bit version which I am running.

The sound doesn't work properly though. In REALTEK control panel you get a picture of your set up (as shown below) so that you can click each individual speaker & see if you're getting sound from them. Well they all work fine when I click them individually in the REALTEK control panel, but when I listen to music, watch videos or play games etc, I only get sound coming out of the front 2 speakers.

Why would this be?



Here is my motherboard - http://www.gigabyte.com.tw/Products/Motherboard/Products_Overview.aspx?ProductID=2551