The automatic audio switching can sometimes fail if a game initializes audio before vorpX can hook. If that is the case, disable “Use built-in audio” in the vorpX config app and change the default audio device manually in Windows.
If you are using the Rift Core 2.0 beta, you should also consider going back to the stable 1.2 build by signing out of the beta channel in Oculus Home. The beta introduces additional audio related glitches, which will hopefully get straightened out before Oculus releases Core 2.0 officially.