AC3 Explained in Plain English

AC3 is an audio codec. If you download a video file that uses AC3, you will probably not be able to hear any sound until you install the free ac3Filter codec on your system. Once you install it, you will have an “AC3 Filter” control panel, in which you should raise the Master Gain setting. Otherwise, some videos you download may not play at sufficient volume. Newer versions use a config application instead of a control panel and have automatic gain control.

On the Macintosh, use the free QT AC3 Codec.