Dell Precision 7710 - no Audio at all

My Dell Precision 7710 is 5 months old. One week ago the audio on my Windows 10 disappeared - the speaker icon got a red cross. Clicking on it I get the message that no audio speaker is connected. 

I reinstalled the drivers - tried three types: from Dell's website, Nvidia's and directly from Realtek - no result. 
Then reinstalled Windows with drive_C formatting: still no sound. 

I know that my laptop has a built-in Audio from Realtek. But in the Device Manager in the list of sound devices there is no such device: just "NVIDIA High Definition Audio" (in case of Nvidia's driver was installed) and "Audio Device High Definition Audio" (in case of Dell's standard driver) - both related to the HDMI output. No mentioning about Realtek Audio. 
It looks like Windows 10 doesn't recognize Realtek audio hardware anymore. 

I launched device test from Dell's Support website - the results said the system is alright, no problems. 
Thinking that probably this is a hardware issue I also launched the ePSA diagnostics (pressing F12 on the startup), but the test hasn't found any hardware problems. But when I clicked to audio test there (there was something about test beep) - there was no sound from the speaker.
But Windows 10 still doesn't see any sound device (except that mentioning about HDMI). 

Here is 3 screenshots:
https://imgur.com/a/ZgVYT

What's the problem with it and how to fix it? 
Thank you. 

UPD.: Also I've made a specific tests of Microphone and Sound card from Dell's website, but got the message that there is no any input or output devices on my laptop (except HDMI):
https://imgur.com/a/HMvDb