Our website provided a free download of AC3 Codec 0.4 for Mac. The application belongs to Audio & Video Tools.
From the developer:
AC3 Codec is a QuickTime component which allows the playback of AC3 audio in AVI files. The component uses the liba52 library for the AC3 decompressing, you can get more information on this library at http://liba52.sourceforge.net/.
You may want to check out more Mac applications, such as AC3-DTS Killer, TryToAC3 or AC3Jack, which might be similar to AC3 Codec.