It's August 25, 2014, the Google Cast option to "Cast entire screen" is still "experimental", and it will NOT cast the audio of a video. That's at least 10 months in this state. The future doesn't look very promising.
The ONLY reason I purchased Chromecast was to be able to play video from a PC on a TV in another location, using any video player I choose. It's a good thing this product is reasonable cheap, or I'd be even more upset!