Ok issue solved.
Michaa7, your last post helped a lot.
The internal mic worked by adding to /etc/modprobe.d/alsa-base.conf this line
options snd-hda-intel model=asus-mode4
(i have a lenovo notebook by the way)
Now, the tricky part was that when adding this line, an "internal mic" bar appears. Which is not working. The real internal mic is actually assigned to the "Digital" bar, something that was mentioned to the link of your last post.
Thanks a lot for your time and effort, much appreciated.