AC3 Codec for Mac
The AC3 Codec is an open-source Quicktime component designed to enable the playback of the AC3 audio from AVI files. All you have to do is to move the component to your 'Library/Quicktime' folder and the plug-in makes sure you'll hear the sound in AVI files.
The AC3 Codec works seamlessly and features no user interface. In addition, from the moment you copy this lightweight (17 KB) component to your Quicktime plugin folder, no configuration is required.
AC3 is Dolby Digital's technology for the audio component in AVI, XviD and DivX files. This plug-in was developed for users of older versions of Quicktime, which did not come with AC3 support. However, newer versions of Apples' popular multimedia framework fully support Dolby Digital's technologies, thus this plug-in might not be useful nowadays.
Moreover, you will need to install version 5.1 of DivX, which is quite old as well. All things considered, the AC3 Codec might not help you in any way if you use newer versions of Quicktime, but it does solve the audio issues in AVI files for older versions of the multimedia framework.