You may have to purchase AirFoil which does what OS X's audio controls do - maps the Airport Express (and Airplay devices in the network) to an Audio Device in Windows Control Panel - Devices/Sound.
That way JRiver will just see it as another audio device to play to.
https://www.rogueamoeba.com/airfoil/windows/
That way JRiver will just see it as another audio device to play to.
https://www.rogueamoeba.com/airfoil/windows/