After installing a new Geforce graphics card with native HDMI audio or Displayport audio, my PCs onboard audio disappears.

Certain computer motherboards which feature an onboard audio controller will disable the onboard audio controller if it detects the presense of another audio controller installed in your PC. Newer NVIDIA GPUs such as the Geforce G210, Geforce GT220, etc. feature an onboard HDMI and DisplayPort audio codec. This allows the graphics card to send audio as well as video to a connected display via HDMI or DisplayPort if your display can receive audio through these inputs and if your graphics card came with either of these display outputs. 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.