I'm no longer developing Soundcast. I strongly recommend a newly available commercial solution that works flawlessly and includes all of the features people were asking for Soundcast:
Automatic synchronized playback using as many devices as you wish (Chromecast, Apple TV, Airport Express, Bluetooth, iOS, Linux, etc)
Sends cover artwork for the currently playing song to the TV
Allows to optionally stream only a specific app's audio (eg. Spotify, Apple Music), so system sounds won't interfere with music
Allows changing the streaming volume (and even linking it to the system volume)