With the release of Mountain Lion, many OS X 10.8 Lion users only to find that when they play MKV, AVI, MP4 or any video files with AC3 stereo audio, AC3 5.1 surround with Perian Player, no sound is played, only the video portion is played.
Interesting thing is if you convert the MKV, AVI to MP4 or M4V and passthrough the AC3-audio, the QuickTime 7 can play the file fine.
In fact, it is not strange at all if you get to know that Dolby Digital audio AC3 is a paid audio codec. QuickTime has got an ac-3 decoder from Apple, but other apps like VLC, Perian can’t use the AC3 codec due to copyright issue.
How to Fix Perian AC3 Support on Mountain Lion
Then is there no way that Perian can play AC3 audio codec?
The question is definitely yes. By doing some tricks, the Perian can still play AC audio codec on Mac Mountain Lion. But only in multi-channel (stereo), no 5.1 surround sound (6 channels) can be supported right now.
Step 1: Donwnload the a52codec.component
Step 2: Unzip A52Codec
Unzip it. It should replace the one in /Library/Audio/Plug-Ins/Components or ~/Library/Audio/Plug-Ins/Components (the Library folder in your Home folder).
Step 3: Select “multi-channel output” in perian preferences.
If you wish play MKV AC3 in 5.1 surround sound on Mountain Lion, you will need to convert MKV to MP4 AC3 5.1 or MP4 AAC 5.1 by some professional tool.