Thank you for this last tip. This solved the problem. Actually I didn't exaclty need the KDE sound set. I just wanted to use the default sounds to see if everything was working correctly. So it seems it was a use flag problem. I have to say that I actualy had the ogg use flag in my make.conf:
ogg - Adds support for the Ogg container format (commonly used by Vorbis, Theora and flac)
But apparently, it was the wrong flag. Well, thanks to everyone who helped! Thank you really!
Valerio