Cast to UPnP/DLNA on the Google Play store can help. 
Ensure you have WiFI started and connected to your local network.
Install, run and simply click the START GOOGLE MUSIC button, you may get a warning button like the screen on the left, take note a click "OK"
Google Play Music will start on your device, and then click the usual Cast button on the top left.
When you do the options to select a device to stream to should list contain all your UPnP/DLNA clients on your network.
On a side note of using ROCKI, you can find out more great ways to utilize your hardware over at the ROCKI community forums.