In this tutorial, we present a method for individually assigning audio sources to different audio outputs.
1. Right click the sound icon in the taskbar and select Playback Devices.
2. Make the destination audio output the default device.
3. Launch the audio sources you wish to assign to the output you just made default.
Repeat steps 2 and 3 for each audio output you want to use.
Open up the volume controls once you're finished tinkering, and you should end up with something like this:
The trick: Each time you open an application, its sound will be routed to whatever output you have currently selected as the default. This pairing between source and output is "sticky." Changing the default output does not migrate any of the sources to the new default output.
Microsoft should make this easier. A 2D grid of audio inputs/outputs would be optimal, but perhaps the easiest thing to implement at this stage is a "move to" contextual menu item, which moves the selected audio source to a specified audio output.