"I downloaded an AVI movie and Windows Media Player shows the video but I can't hear any sound. Is the file corrupted or am I doing something wrong? Other AVI movies play just fine."
Getting video and no sound or sound and no video is more common than you might realize. This problem is almost always caused by one of two problems. Either you don't have the required codec installed on your system to decode the audio portion of the file you downloaded. Or Windows Media Player isn't properly associating the correct codec on your system in a way that makes playback possible. The tricky part is the sheer volume of codecs out there and the number of poorly programmed codecs that potentially cause your system to crash. So how do you get the file to play?
Which Codec Do You Need?
The first step in correcting playback is finding out what codecs are required. AVI files typically require one codec to display the video and a different codec for displaying audio. My favorite tool for checking AVI required codecs is GSpot, which is a free download.
GSpot quickly troubleshoots potential video issues by identifying things like the length of the movie, which it compares against the expected length embedded in the file to determine if you have a complete file. GSpot tells you which codec is required for video and audio and how many of the codecs on your system are capable of playing back a file. A Render feature simulates file playback to determine if the software can render the file correctly.
If you find that you are missing a codec, you need to locate that codec online and download it. Some compatible codecs are listed at the Microsoft run WMPlugins.com. Others must be tracked down from one of the various online repositories. It's very common for codecs related to DivX (including XviD and 3ivX) to have issues, particularly because of the way audio and video are paired during the encoding process.
DivX has their own official codec package. The 3viX codec pack, which is arguably more reliable than either, is available from 3viX for about $7. Tracking down a decent XviD codec can be a little more complicated because the developers don't offer a pre-compiled binary and the options are widely varied. DivX Digest currently offers a decent range of options.
One common problem in AVI files where video plays and audio doesn't is a lack of compatible AC3 codec. The free AC3Filter available from Sourceforge does the trick quite nicely in almost every case.