This Qt App can control settings of Stereo Mix in Windows OS: enable/disable device, change volume, playback device and power management (mmsys.cpl).
Used COM and WASAPI without third party libs.
COM Interfaces: IMMDevice, IMMDeviceEnumerator, IMMDeviceCollection, IAudioEndpointVolume, IPropertyStore, IPolicyConfig.
Note
EDataFlow enumeration:
https://learn.microsoft.com/en-us/windows/win32/api/mmdeviceapi/ne-mmdeviceapi-edataflow
Recording devices (mmsys.cpl):
[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\MMDevices\Audio\Capture]
Important
The COM interface: IPolicyConfig is undocumented and the GUID can be changed in Windows OS.
For debugging, download symbols mmsys.pdb.