If you’ve downloaded AVI files and tried to play them in Quicktime, only to discover that they don’t play with any audio, then try installing A52Codec. It provides a codec for AC3 audio in quicktime, with the following features:
See the A52Codec website for more information. Once you have downloaded the A52 Codec you should have no problems playing these files.
In addition to A52Codec, you could alternatively have a look at Perian – an open source project which provides AC3 support as well as a large number of other codecs:
They do call it the Swiss Army knife of Quicktime components! Head over to www.perian.org to read more.
Thanks to Graham Booker (the author of A52Codec) for the updated information provided in this post.