No matter what I do, audio is routed to my monitor instead of to my Rift.

I've tried:

Making sure that the Rift Headphones are the default audio device
Alt-Tabbing out of the game and setting the Rift Headphones to the default.
Changing sound configuration, etc.

The game puts up a windowed image on my monitor mirroring the Rift's display for both eyes.
It appears that as it does this it routes the sound to that monitor as the main output device.

Other programs don't appear to do this. Usually my monitor screen does not show gameplay, nor VR interfaces, etc. all video and audio output go to the Rift once started.

Anyone have any ideas?