You might try going to the control panel and device manager. Go to the sound card. Click on it. There is a tab for driver and there is an option to roll back to previous driver. This will load the original driver back for that device. It's worth a try before going to Realtek site. ( It's a bone...
I am having a similar problem in Vista with my speakers. My front set will not work ( though Windows says they are fine ) and the rear speakers work just fine, thank you. If I switch the cables, the former front speakers ( now rear speakers ) will work and the former rear speakers will not work...