Hello. You should be able to output HDMI audio when connected to a device capable of receiving HDMI audio. See the HDMI AudioFAQ for tips on setting it up.

You might need to update your video (graphics) drivers. I would expect to see this in your Device Manager:

"Intel Display Audio" is the name of the HDMI audio section of the Intel HD graphics driver. You can get the drivers from the Latitude E6420 support page. Be sure to select your Windows 7 version -- 32-bit or 64-bit -- by using the blue Change OS button.