So I’ve been having this problem with certain video files downloaded from the Internet: the audio fades in and out for no apparent reason. Usually I just delete it and download another version but today I was determined to fix this.
The symptoms: the audio fades in and out, and peaks the first split second of voice audio, and then fades down after that. And this only happens in Windows Media Player. I figured it has something to do with the audio codec and something with surround sound because it swaps the voice audio and background audio throughout the playback. The first step was to find a program that shows the codec info of each video file. Microsoft Windows Media Player’s File>Properties just doesn’t show enough information.

My searching led me to this awesome application called afreeCodecVT. It’ll show you everything about a file’s codecs, both video and audio:

Codec : FAST Multimedia AG DVM (Dolby AC3)
Flags : None Set
Quality : 0
bps : 448,000
Codec Code : 8192
Channels : 5
SamplePerSec : 48,000

AfreeCodecVT codec info helped me track down the problem format to be AC3 (Audio Code: 8192, Codec: FAST Multimedia AG). It turns out that there are two AC3 codecs out there. One is the more common AC3 filter is available from SourceForge (ac3filter_0_70b). The second is the Dolby AC3 Audio (8192) codec, the one I was missing. After some Googling I found and downloaded AC3 Filter 1.11 (ac3dec124b). Problem solved!