Hi eray96,
I think you got mountain lion, don't you?
Here the solution from user R08 that worked for me, because I had the same problem.
AC3 should still play under Mountain Lion in QT X so I'm assuming your problem is with QT 7 and file formats not supported by QT X.
For me this was happening with .mkv files containing AC3 audio played through QT 7 using Perian (which as I understand it is only getting one more release which may or may not fix this issue).
Thankfully like all things it's easy when you know how, this is what worked for me.
Download and extract the A52Codec.component file and replace the one in your Mac's /Library/Audio/Plug-Ins/Components/ directory with it.
Go to System Preferences > Perian and in the drop down box next to Audio Output: choose Multi-Channel Output, ignoring the warning which will be displayed.
Close System Preferences.
This fixed the problem for me in that I can now play my files (H.264 video & AC3 audio in the .mkv container) using QT7 and also export them with the video passed through as is and the audio transcoded to AAC for my Apple TV.
Hope this helps...
deltaguru