Hi Toronaga,
Ok, that's a strange problem. I can only think of a couple of things which may cause something like this.
The first thing to check is that the volume control on WMP is not too low.
The second possibility is to do with multiple sound cards. If you have more than one card have you made sure that the sound in WMP is being directed to the correct one?
Have you checked that WMP can play other audio files correctly? (ie. Files not created with Golden Records).
When you try and play these files with WMP, what length does WMP display them as having?
Regards,
Ben