I went into advanced audio and changed the hdmi input to digital and it worked :) So i now have sound on my chromecast via older Panasonic HDTV
Im not sure if this resolves the Nexus 10 hdmi sound issues
There is still an audio issue with panasonic via HDMI, no audio via chromecast.
Pretty sad this still isn't resolved, oh well off to ebay to sell my chromecast