QT 10.0 cannot play ac3 files

http://perian.org/#download

Yes.

With Perian installed you can play them in Quicktime Player.

Otherwise only VLC.

Am I missing something?

That is difficult to say not knowing what you do or do not already know. Basically, AC3 audio is not natively supported by QT. As QTKirk indicated, you need additional support and Perian is one such option. However, how you wish to play the AC3 audio will determine what kind of support you will require. Normally, apps like A.Pack, Compressor, or the Apple DVD Player deal with AC3 audio. For instance, the DVD player (like VLC) can either down convert 5.1 AC3 content and play them as stereo through your computer's built-in speakers. Or, it could pass the undecoded 5.1 AC3 signal to your optical output and let you decode and play the audio externally through an external amplifier/speaker system.

For instance, QT, with the proper software support, will play the 5-channel AC3 files, as downloaded, through my Logitech Z-680 amplified speakers system. However, to get MPEG Streamclip to load such a file, I first have to re-wrap the data in an MOV file container. (I.e. MPEG Streamclip is a QT medial player that has built-in AC3 audio support for mixed down playback through your system speakers just as VLC and the DVD Player do.)

Just one question in my mind, since VLC has installed many codecs for different media types, can't QT uses those codecs for playback? Is there any sort of restrictions/limitations/settings?

VLC is an open source, multi-platform project. As far as I know, the codecs are built-in/only accessible via the app itself and not written as Mac compatible QT components which are/can be stored in any of the various "Library/QuickTime" folders as Apple's own and all compatible third-party codec components are stored for access by the embedded QT structure/called procedures. These locations are essentially "hardwired" for use and there are no user settings or options to allow/tell QT to look elsewhere. You could, of course, try writing your own component—sort of a "go-between" switcher for QT and VLC but I'd hate to be in your shoes when it comes time to write updates for every QT and VLC update that changes some nuance of data flow for a particular codec in the VLC arsenal; troubleshoot compatibility issues for QT, VLC and all possible other third-party components for which a Mac might be configured; then there is the question of QT 7/QT X dual 32-/64-bit compatibility issues; the issue of providing internal "legacy" transcoding support where it doesn't already exist; etc., etc., etc.