No Audio with media player

Hi.

My father-in-laws computer has a pretty strange problem. We can play CD's and DVD's fine with all programs, mainly Itunes. But once we use Windows Media Player we get no audio at all. We can listen to the audio in WMP while playing the demo songs that it came with but if we use our own DVD's or CD's we get none at all. We get the following error message.

( Media player cannot play the file because there is a problem with your sound device. There might not be a sound device installed on your computer, it might be in use by another program or may not be functioning properly )

I have played most of his DVD's on my vista machine and they work fine. So it is not the medium that is the issue.

I check all audio drivers and they are fine, I added a new set of codecs and nothing changed. Audio works perfect with ITunes and the other music program he has, but just does not work with WMP.
He has Vista home premium 32 bit.

So what may be the problem?.

Thanks

Make sure itunes is still not running in the task manager, on xp if you just click on the x to close it will still be running.

Sorry I should have stated in my post that I only just installed Itunes tonight as WMP wont play audio. My fault.

But yeah, WMP worked with audio for about a week and then just died one day.