I'm having a similar problem and I've already updated all the drivers. I'm using the Logiteck X-540 speakers with the Realtek HD Audio Manager for my Gigabyte motherboard. It's a 5.1 speaker system but it doesn't give me the option of selecting the Rear Pair when using the 5.1 Speaker configuration. Like some people above I have to use the 7.1 config but that's giving me playback issues with some games. And yes I do have the cables plugged into their correct ports.
I've read all the posts and nothing I've seen so far has managed to help my problem. Does anyone have a fix for this?
I'm currently running windows vista