"The MPEG-2 Playback Component does not support audio playback for MPEG-2 files containing PCM, DTS or AC3 audio."
There are a couple of workarounds though.. .and this may even save you a few $$$. The option I use is to download the Squared 5 MPEG Streamer converter
This is a standalone application which will playback the audio! It also allows you to edit, (unlike QTime Pro with Vob files) and export to many other common file types. Not only that- but it can also playback full screen! So if you haven't purchased QT Pro, you can play back any format QuickTime can handle using this application, and the best news is... it is FREE!!! The only catch is that this application will need the you to have purchased the Apple Mpeg2 component plug-in. Talk about false advertising... you need to buy the Apple plug in in order to play VOB files, however the plug in won't playback audio so you need to download a free 3rd part application which actually works and does what the original plug in should do anyway... weird....
Since I am on the topic of Quicktime plug ins and codecs, I have mentioned this before but if you haven't already done so download Perian - The Swiss Army Knife of Quicktime Codecs.