I know you've already reset the NVRAM (what you're referring to as PRAM), but the first thing I'd try is to reset both the NVRAM and SMC in that order. Before following the steps below, ensure you have no external hardware connected and make sure you use the built-in keyboard.
Reset the NVRAM
Your Mac uses Non-Volatile Random-Access Memory (NVRAM). Here’s how to reset the NVRAM on your particular MBP:
- Shut down your machine. Yes, a full shut down, not just logging out.
- Press the power button and then press the commandoptionpr keys. You have to make sure you press these keys before the gray screen appears or it won’t work.
- Hold those keys down until your Mac reboots again and you here the startup chime for the second time.
- Let go of the keys and let your Mac reboot normally.
Note: When you log back in you may need to readjust some of your system preferences (e.g. speaker volume, screen resolution, startup disk selection, time zone information, etc).
Once done, proceed to resetting the SMC as well.
Reset the SMC
To reset the System management Controller on your particular MBP, follow these steps:
- Shut down your computer
- Keep the power cable plugged in
- Press at the same time shiftoptioncontrol (on the left side of the keyboard) and the power button and hold all these down for at least 10 seconds
- Let go
- Turn your computer back on with the power button.
After resetting both the NVRAM and SMC, use your Mac to determine if you still have no sound output.
If it doesn't, then it's time to test your hardware with Apple Diagnostics.
Run Apple Diagnostics
Follow the steps below to run Apple Diagnostics:
- Fully shut down your MBP
- Restart your MBP
- Immediately press the D key and keep it down until you see the Diagnostics screen appear
- Wait for Diagnostics to finish (this typically only takes a few minutes)
- Once complete, one of two things will appear on the screen:
- a No issues found message
- a brief description of any errors found plus further instructions
- If the diagnostics test does find errors, take a note of what they are
Note: If pressing and holding the D key at Step 3 doesn't work, start again at Step 1 and, at Step 3 press and hold both the OptionD keys instead. This will try and run diagnostics from the internet instead, so you will need to allow more time for it to complete.
Regardless, take a note of what happens and let me know how you went.