No HDMI audio output option will appear unless a suitable audio input option is also available on the input device. Make sure it is configured to receive audio from the port you are using. Not all receivers detect and switch automatically to the correct audio port.
Also, check the unfortunately named "Audio MIDI Setup" program in your Mac's Utilities folder. It will show you multichannel audio options for multichannel audio sources.
One more thing... make sure the adapter you are using specifically describes it as a "Thunderbolt to HDMI adapter with audio support". Earlier Macs had Mini DisplayPorts with no audio. Unless you buy the correct adapter audio won't work.
Correct adapters for your MBP are available for less than $10.