The most likely fault is that even though Windows installed drivers for your card, they are inappropriate for your particular card. For example, on my system, the card supports optical and analog output through the same jack. Windows 7 installs a driver for it by default, but that driver enables digital optical output all the time, and thus does not work with analog output and with the built-in speakers in my laptop. Go to the manufacturer of card's website or elsewhere on the internet where you can download drivers for your card's chipset. That should remedy the issue.
i always get my sound driver by running windows update.(soundmax)run windows update and see if sound driver is listed there.this is the first thing i try before going through the trouble of finding out the type of sound device and going to the manufacturers website.if that doesnt work,i use pc wizard(which is free to download) to find out what devices and hardware i have.hope this helps,have a good one.(you have to click on the little arrow next to sound video and game controllers to see your audio controller)
Just for the info, since the first release of W7 I had no problems with Realtek drivers made for Vista.
Had to download creative drivers for RTM too , wich was weird since it always auto installed them in the RC etc .