VLC bears no relation to QuickTime; it uses its own set of codecs and file parsers. So you should expect differences in the way QuickTime and VLC handle certain files.
However, if you have MPEGs that play sound in VLC but not QT, I can almost guarantee you that they're not real MPEGs. Windows users often misname any ol' movie file ".mpg", even if they're really mov, wmv, avi, etc. It's probably really an avi. If I'm wrong and it is in fact an MPEG, it means that it's somewhat corrupt. QuickTime is notoriously non-error-tolerant, whereas VLC is very error-tolerant.
Ok. Thanks alot. I downloaded a "movie trailer" from apple to test Quicktime to see if it was something 'wrong' with it (corrupt). But it played fine. So I've downloaded those other codecs and that should fix it.
Thanks for your explaination!