I don’t know if you can get away with that, but I do know what you can’t do with “the tools you have”.
The first approach would be to - having both output devices connected – create an Aggregate device that encompasses both.
Let’s say one device is called: Headphones and the other is Display.
The initial approach would be to create the Aggregate device in your Audio MIDI Setup panel, add Headphones and Display as devices, mute Headphones’ left channel and then Display’s right channel. Then happily select this new Aggregate device as your sound output and be super happy.
Sadly that doesn’t work. (I’ve tried it). Apparently the clock synchronization between the “devices” doesn’t happen and OS X doesn’t really care, therefore only one device outputs audio. (Sad, but true).
(more information on how to create an Aggregate device that can be found on this Apple Article)
What to do?
There’s hope, but I haven’t tried this solution.
The idea would be to create an aggregate device of your two sources (Display and Headphones) like we did before, then install JackOSX, set its preference to your new aggregate device, start the Jack’s router, set AudioMIDI Setup to use Jack for I/O, open an app to play your music and connect its output send channels to whatever system playback channels you want in Jack Connection manager.
That might work and I suggest you give it a shot.
More simple solutions, might involve cabling, Y-Splitters and weird things which I don’t think you want to dive into (specially given that all the audio is Digital and internal, you’re not talking about external speakers per se).
Finally, although it doesn’t provide simultaneous playback, changing sound sources is very simple with a free utility like SoundSource from your menu bar. (I do it all the time to switch from headphones to speakers).
Hope something of this makes sense.
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.
Best Answer
From your question it is not clear if you still have a question, but let me give you a answer anyway.
There is nice little app on your Mac called Audio MIDI Setup (in the utility folder)
Open it and use the setup to for example have multiple audio output, or set up and Aggregate devices and so on to your desire.