BubbleUPnP 1.7.8 will be shortly available on Google Play.
It doesn't contain Chromecast support yet which will be for the next update.
But it includes an exciting new feature called Audio Cast, allowing to capture and play audio from most third party music apps on rooted Android 4.1+ devices running the Xposed framework
With Audio Cast, you can play audio from Spotify, Deezer, Poweramp and many more apps to your renderers, using these apps for playback control and hardware volume buttons for renderer volume control at the system level. It handles exactly like if you were playing local audio.
Check the video to see Audio Cast streaming Spotify to XBMC and Xbox 360, and a bit of Poweramp!
The full setup and usage documentation can be found in the v1.7.8 app, in Settings > About/Help > Tips > Audio Cast.
Xposed is required as BubbleUPnP includes it own Xposed module for the audio capture.
This feature has been really tricky to implement so I hope it will be useful!