Thunderbolt Audio is not an option in Sound Preferences

I have just had my rMacBook Pro returned after repair (Wonderful Apple Customer Care btw).

I like to out put the audio through the Thunderbolt Display but when I Option Click on the Volume Icon or go through the Sound in System Preferences, there is no option to select Display Audio from the list as there was prevviously.

I have tried restarting the MAcBook and Display, removing power cord etc.

Thanks for any advice that might help.