Enter "Virtual Audio Streaming," an app from virtualaudiostreaming.net This app enables you to pick up sound from any application on your Windows system and pipe that audio into another application. It runs as a service, so the controller is in your task tray. When you open it, this is what you see:
On the left, set Virtual Audio Streaming as the default audio device (alternatively, pick VAS as the audio device in your application, such as SDR#). On the right, set it as the default sound recording device (or, alternatively, select it as the sound input device in the target application like Qtmm).
The tool installs easily--as with most low-cost applications, it can be bundled with other apps so pay attention when you're running the installer (if you don't want any other apps installed). Once running, it isn't noticeable at all and seems to have no effect on the host. A quick peek in Task Manager shows it takes on average between 2% and 4% resources:
From here on out, it's a snap to run with. If your source and target apps allow you to pick custom sound devices, you can run Virtual Audio Streaming without impacting any other audio settings (this is handy for me, because I spend up to 75% of my day in online meetings using a variety of conferencing tools). Overall, the install was easy, configuration is a snap, the tool is very flexible and it's had no noticeable negative effect on my computer.
Enjoy!