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)
You can get it here