Not sure, but investigate this. You may NOT want to have your graphics card do audio output for you. The main reason for outputting audio from the graphics card is so that the sound can be sent on the HDMI cable to the display device that may also contain your speakers. But very often the audio output ability of the graphics card is limited to 2-chanel stereo, assuming that's all you monitor has.
If you need a more sophisticated audio signal to be sent out to a high-quality audio system, maybe you should be connecting via a multi-channel electrical or optical audio output from the audio system built into your mobo. If you do this, then you need to make two adjustments. The first is to Windows. Windows can use ONE audio output device at a time, and yours probably is set now to output from the chip in your video card. On Win XP (more recent is similar), click on Start ... Control Panel ... Sounds and Audio Devices. In the new window click the Audio Tab, and it will show you three rectangles for different functions, each containing a drop-down chooser slot. The top one is Default Device for Sound Playback, so click its down-arrow and choose the audio output system on your mobo (often Realtek). You can make similar choices in the other two rectangles if you want to. Click on OK at the bottom and back out of the menus.
Next you need to open up the Audio Manager (or similar utility) that came with your mobo for its audio system and configure it to output the sound signals your external system requires - how many channels, etc. This should get you advanced audio on that system, separated from the video still going out on your HDMI cable to your monitor.