I've configured my soundcard properly: all sound settings are ok;
the only thing that's not working correctly it's the sound for the microphone: the volume (even if at 100% in alsamixer) is very low (I cannot use Skype or other VoIP software). Any suggestion?

If $alsamixer is set up properly and saved with $"alsactl store" to make the settings permanent( a more manual way that I configure it on a slackware system), then it might be something related to the desktop environment your using, i.e. KDE or Gnome or what-have-you.
It might not be THE solution, but its all I can think of off the top of my head.

already done... the strange thing is that if I run $alsamixer and after I press tab I have the microphone set to capture, but I cannot adjust the volume, not even mute or unmute it...

Look for a "Mix Boost" option in alsamixer. This increases the microphone's sensitivity.

where it should be?

I suspect that the existence of this option depends on your hardware, and perhaps even the name.

Be aware that some mixer apps (the gnome mixer is one of these) only show the basic controls. In the case of the gnome-volume-control program there are preferences you can set to show other controls, including in the switches tab.

Another option is using the alsamixergui tool. You may need to extend the window horizonally to see all the controls, or scroll accross to see the setting in question: Mic Boost (+20 db). You can see it highlighed in this screenshot.

Have a read of all the control names - there may be another name for similar functionality for your device.

OK, I haven't. But I'm sure my card is able to do it (on Windows it does, and the chip supports it). Maybe I can edit some .cfg file to add this option?

All that should be necessary is finding the option in alsamixergui / gnome-volume-control / your favourite mixer app.

nothing like mic boost there...

In that case I don't know what might help.

What card ans chip does alsamixergui identify your soundcard as (it shows these in the panels near the top of the window)?

It identifies the CA0106, and I have the CA0106-DAT-LF

I googled about a bit, and it looks like there is some bug associated with that driver and the db gain option.

http://www.spinics.net/lists/alsa-devel/msg03499.html

Upgrading the driver is a pain, but at least you know you're not going mad

thankyou very much, I'll try and see what It will came out

::EDIT::
well, it seems that is not my ALSA version (I have a newer one) and I don't think I'm able to do that, It seems I have to modify the configuration file and compile ALSA again... I'm not sure I'm able to do that...

no one can help me?