The problem was that this on-board sound card is surround and, when in surround mode, the mic mini-plug input becomes Center /LFE output.

I fixed it this way:

1) Open “alsamixer”
2) Go to “Mic-In Mode” bar
3) Check that in “Item” (top-left corner), it reads “Mic-In Mode [Mic-In]”
4) If not (probably “Mic-In Mode [Center/LFE Output]“) hit up/down arrow keys until “Mic-In Mode [Mic-In]” is selected.
5) Check that mic is not muted. Go to “Mic” bar and check that it is not muted “Mic [Off]“. If it is, hit M key, you’ll get “Mic” only.

That should be it.