Hi,
Check out this page, which describes how to enable Stereo Mix. I guess you already know how to do this, but one thing I noticed there is that you need to make sure that "Show Disabled Devices" and "Show Disconnected Devices" are enabled. It could be that you just need to do that to see your Stereo Mix device. If that doesn't work then I guess you do need to find new drivers, but I don't know which ones I'm sorry.
If you can't get this working, try our streaming audio recorder, SoundTap. It can record audio sent to any Windows output device, so you may be able to use it to achieve the same result as Stereo Mix.
Alex