Through a lengthy I finally found this:
Which hinted to the fact that "Heaphone Mic Boost" is to blame for this. Issuing:
$ amixer -c 0 set 'Headphone Mic Boost',0 1
makes the problem totally disappear for me. But why it depends on which kernel has being booted?
Hope this helps somebody out there; I found this problem very obnoxious.