How can I troubleshoot no audio via Thunderbolt HDMI connecting to HDTV?
That model of MBP supports audio through that adapter, so you should be getting sound. It could be any number of things. I think your problem is on the TV side, but Here are a bunch of things to try: 

First, make sure you're connected to your TV and getting an HDMI video signal to the  video input you've selected.

Go to your MIDI Audio Setup again and make sure that your thunderbolt output is set to "Use this device for sound output." Highlight it, and go down to the setup (gear icon below) to set it.

From your Apple Menu, open your System Preferences and choose Sound. Try setting your "Use audio port for:" to Sound Input, as it may be hogging the audio out channel and not letting your thunderbolt use it. Make sure your system volume is turned up there too.

If you have various sound drivers installed and showing up in Audio MIDI Setup, just leave them alone for now, and DO NOT use any aggregate devices. Select your thunderbolt output and make sure the volume sliders are turned up. You should see the speaker icon next to that audio output selection too. We're not interested in "Sound Effects" in this case, so don't worry about that.

Recheck ALL your connections.

Make sure your volume is turned up on the MBP.

THe content you're trying to play probably has a volume control of it's own too. Make sure it's turned up.

And make sure that your TV volume is up, and not accidentally on Mute.

IF your TV's set up to play audio through your amplifier (surround, etc.) make sure your amp is on, set to TV as it's input, and volume is up. Don't turn these all up to max! When you find the culprit, you don't want to blow anything out.

Make sure your TV isn't in SAP mode.

Check to make sure you don't already have an audio source plugged into the audio jacks for that input on your TV. HDMI only on that TV input.

Check the setup options for Audio in the app you're using to play your content. If it's a movie, for instance, and you're using VLC as your player, check your Audio Device setup in the Audio menu option.

Make sure your TV isn't setup to expect audio from it's Stereo input jacks, as if you were connecting via component video and audio. Light up the menu options on your TV, and just double check your Audio options settings. Maybe that input port on your TV is pre-set to expect only separate stereo jacks on that input. Just a thought. You might try another HDMI input on your TV just as a test.

Try a different HDMI cable if you've got one.

That's all I can think of. I have a feeling one of these will solve the problem.