I've been using this laptop since April 2016 (Linux version), this summer I had the same issue - no sound card detected, but after bios restore to defaults it appeared again. Now restoring to bios defaults doesn't help - no audio controller in bios and only dummy output in OS. Are there any chances it's not motherboard broken a few months later after the warranty has expired?
When you say no audio controller in BIOS, are you referring to the Device Information section under System Information? If so, can you try entering the diagnostics (press and hold Fn whilst powering the system on) and running the speaker test? If there is no sound from that, it's not looking too good I'm afraid.
All I could then suggest is to try updating the BIOS: