No sound over Mini DisplayPort to HDMI Adapter

I bought a Mini DisplayPort to HDMI adapter (http://www.cables2u.co.uk/mini-displayport-hdmi-socket-p-844.html) and this adapter should allow the transmission of both video and audio. Video works fine but no audio is transmitted when I connect my MacBook Air to a HDTV, and there is not even an option to do so under System Preferences > Sound > Output (Internal Speakers is the only option available). Any suggestions?