After rebuilding my Mac Pro with Windows 7 on an SSD (more about this later), Stereo Mix went missing.
To re-enable it, I ended up changing the audio driver to the Microsoft High-Definition Audio drivers, then back to the Realtek drivers:
- Control Panel / Device Manager
- Browse to Sound Video and Game Controllers
- Choose Realtek High Definition Audio / Change (you’ll need an admin password at this point)
- Update driver / Browse / Let me pick, and choose High Definition Audio Device.
- Let it finish, then go back in again but at the last step choose Realtek High Definition Audio. This time I found it needed a reboot.
I assume this updates you to the drivers that came with Windows, rather than those that came with Boot Camp.
After the reboot, Stereo Mix is available. You just need to enable it under Control Panel / Sound / Recording devices, right-click, Show disabled devices, then enable it. You can set it as the default so you can record things in Audacity etc.