I can't believe i'm the only one out there having trouble with AC3 audio via Quicktime.
So for anyone else who with troubles with .avi files with no sound goto http://www.macupdate.com/info.php/id/21875
The A52Codec is a CoreAudio component that can decode stereo and 5.1 surround sound from A52 and AC-3 audio. This type of audio is often found in DVD movies. Requirements;Mac OS X 10.4 or later, QuickTime 7.0.1 or later.