Can you tell us more abou the files that don't play? Can you post a short sample? Could you also post a list of plugins
If you have a mono audio file you cannot turn it into a stereo file, there is no stereo seperation information. Check that you haven't got "Force mono" checked in Preferences -> Playback.
One way to create the illusion is to render the track into two channels and then invert one of them. I don't think you can do this on playback in Winamp. Wait you might be able to do this with the Nullsoft SPS plugin. Hang on.