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further information: DirectShow Playback Guide
Enabling the Dolby Decoder function of FFDSHOW.

Dolby Digital (AC3) is the trade name for a series of lossy audio compression technologies developed by Dolby Laboratories, Inc. AC3 actually refers to a specific version (the original and most common) of the Dolby Digital "suite" of codecs. It is commonly used as the audio tracks on DVDs and inside many video container formats such as AVI files. AC3 streams can contain high-quality 5.1 surround sound audio, while keeping file sizes small by using compression on the audio track. Dolby's primary competition in this market is DTS, which can sometimes provide slightly higher quality audio (though with higher file sizes). MC can support both AC3 and DTS audio via DirectShow.

Video files include their audio tracks as a separate "stream" contained inside the file. This audio stream can be in a wide variety of formats, but it is very commonly one of three options: uncompressed PCM (WAV audio essentially), MP3, or AC3 (Dolby Digital). In order to properly play back the audio track you will need to have a decoder that can handle the proper audio type installed and configured on your system.

Quite often, when the audio fails to play back it's because the audio track is in AC3 format. If you are using FFDSHOW (or CCCP) to handle the decoding of your video files, FFDSHOW can also handle decoding of AC3 content all on it's own, but you need to enable the option. Simply open the FFDSHOW Audio Decoder Configuration dialog (Start --> Programs --> FFDSHOW or Combined Community Codec Pack --> FFDSHOW Audio Decoder Configuration) and choose Dolby Decoder from the tree on the left and enable it.

Another option is to use the standalone AC3filter, which also works quite well and reliably.