Ok. I called Creative support and they explained the thing about the
AC-3 decode option. The cards before 5.1 don't decode AC-3 so there is
no option to disable it.

Actually, the Live 5.1 doesn't decode 5.1 EITHER. It's all done in software
in the drivers. What the 5.1 version does have however is an extra stereo
speaker output, previous cards only supported four speakers maximum.

As for my main problem they don't have any idea...
Can anyone help?

Start Powerdvd and run a movie. Right-click and bring up the preferences
window. Select the information tab. Scroll down and see if the program has
identified the digital output and is using it. Go to the sound tab. Verify
the speaker environment is set as SPDIF. If all these are OK and you still
don't get dolby digital you'll have to turn to PowerDVD support, but they're
all indians who know sweet fanny adams about anything. I can't get DD or DTS
out my SPDIF either from my Audigy2 ZS and they couldn't answer even the
simplest of questions. Good luck with them, you'll need it. ^ ^
Fortunately I have a native 5.1 sound system, I just use the analog
outputs, but not everybody has that option. I don't recommend powerdvd
anymore, for starters they "update" the program annually and you'll have to
pay $70 all over again, but the difference is minimal and support is
TERRIBLE. I sure as hell won't buy the next version...