I have two two AC-3 codec's installed in my computer: Nero Digital Audio Decoder and MainConcept MPEG Audio Decoder. I can watch in perfect conditions a movie with ac-3 audio in wmp, but i can't hear ac-3 audio files in wmp.
In vlc i can hear ac-3 audio files but in wmp not, why ?

Have you tried AC3Filter?

Yes i have tried but the result is the same

Verify the installed AC3 decoders with GSpot or Sherlock, can be broken. Also, the Nero decoder can cause errors as experienced so try to unreg it.

Wmp is not great for playing standalone dolby AC3 anyway. Most audio players have AC3 audio available as filter/plugin or you pick up free open source AC3 decoder.

or my favorite, Zoom Player along with one of their AC3 filters:
http://www.inmatrix.com/zplayer/formats/ac3.shtml

*Also, VLC and WinAMP, Creative PlayCenter etc. will play AC3 audio.
Lots of choices.