A quick update if someone else is struggling with a similar issue.
I was able to resolve this issue simply removing the “customization” I did. 
In particolar, I removed the alsa-base.conf and the snd-intel.conf from the /etc/modprobe.d/ directory and the system automatically “restored” the previous situation.
I created them when because the audio was not working correctly, but now it seems working as expected, both speakers, microphone and headphones too.
I leave also a screenshot of alsamixer and how it changed after the file removal, if it could be helpful.
However, the audio was still cracking/popping after suspension. Just restart make it work again.
I saw in another thread to use the zenbook-sound-fix-ux3405ma package but the problem remains.
FInally, I followed this other discussion and it seems it’s working like a charm so far. In particolar, I followed Guillaume Lebigot’s suggestion.
Hope that helps. 