Recently upgraded my Nvidia display card to Nvidia GeForce GT 640 and lost my motherboard audio output. Nvidia suggests the following:
Even if your graphics card does not have a native HDMI or DisplayPort connection, if the PC detects an NVIDIA audio codec on the NVIDIA GPU (Graphics Processing Unit), it may still disable the motherboard onboard audio controller. To correct this, you will need to go into your motherboard BIOS and change the setting "Onboard Audio" from "Auto" to "Enabled". This will force the onboard audio controller to always be enabled even if there is another audio device present in your PC. If your motherboard BIOS does not include this setting, check with your PC manufacturer for a newer motherboard BIOS which may add this option.
System board: 2ADA 1.00
System BIOS: 8.19
How do I proceed?