Hi MR EKIP
It's hard to say why this is happening ... could be one thing or another?
Make sure you have Flip4Mac & Perian installed, these work with QuickTime and give you a broader spectrum when it comes to playing different movie files on your Mac.
Don't be too confused about VLC playing the files without trouble as VLC can handle pretty much anything that is thrown at it ... QuickTime isn't as flexable.
.avi files are a container format and it's what's in the container that QT has to play ... the audio/video formats just may be something that QT isn't sure about.
I'd give
Flip4Mac &
Perian a go.
Let us know how it goes ...