The NVIDIA audio drivers are part of the graphics card driver package. Simply download the driver package for your graphics card here:
The appropriate audio support for your graphics card will be installed. If you only need to install the HD audio drivers and not the whole driver package, choose Custom Install and uncheck the other optional driver packages. Note that the base graphics card driver cannot be unchecked.
If you are using XP, you will need to install a Microsoft OS hotfix first as per this article.http://support.microsoft.com/kb/888111/en-us