I'm on MacBook Pro 17, 2.6 dual core, 4Gb memory, OS 10.5.8
Along with lots of other bugs, this one is transfered from version to version again and again. As far as I remember working on Mac (5 years now) this issue was in all Flash versions since CS or CS3. It was reported numerous times, but nothing changed even in CS5.
If you are a Mac user and trying to export a QuickTime movie with some a bit heavier grpahics, all you get is a completely cluttered and basically useless Movie file. It is something about the way Flash records a movie export "on the fly", instead of reading each frame. On the other hand exporting a PNG sequence works perfectly, so I don't see the reason, why not to implement this in QuickTime export too. Also as far as I know this issue doesn't exist on Windows.
The workaround is exporting a PNG sequence and sound separately and editing them in another software, or opening the SWF in Premiere (which – lucky me! – can read SWF files quite well)...
Welcome to never having your questions answered. Flash is now the red-headed stepchild at Adobe. No one there gives a rat's *** about it. Maybe I don't know what I'm talking about. Maybe the future is some sort of Notepad where I program HTML5 or 6 or 7 or whatever comes next. Goodbye to having an actually usable tool for designers to produce believably cool stuff.
I hope you don't mind I marked this reply as correct, since there is no other correct anwer, and probably would never be...
I remember big discussions about the enormous amount of bugs in Flash CS, even some Adobe people participated, and since then none of them were fixed, new bugs were added, and many useful features were even abolished, so that I had to install all kinds of plug-ins to have those features back.
It's a pitty, that the market is still full of Flash based projects.