Some media players may be able to play XviD video without requiring special installation of the XviD codec. XviD is an open source format, so recent versions of your preferred media player may have automatically included XviD capability without the need for you to do anything.
However, if you are in any doubt, not only if the video simply won’t play or you get sound only, but also for example if the video doesn’t look quite right – weird colours – jumpy video – audio sync problems – anything like that, then there’s no harm in reading on and making sure of XviD installation...
To play XviD video you need the XviD codec installed on your machine. As I say, the codec may already be on your machine. It may come as standard with some media players but not on others (e.g. may come with Real Player, but not by default with Windows Media Player). In any case, if you find your media player is not handling the video, then Windows users should install the XviD codec (the latest stable binary) from the following link - Koepi’s XviD link
For more detail on XviD and for latest developments see Koepi’s XviD link and the XviD official homepage (though note that the homepage has source code only - not installer versions).
The version(s) included on Koepi's XviD related website will support play of all XviD video (if you're running Windows). This should always have the latest codec version if you want to keep up with new releases. You should still be able to play video encoded with an older release if you install a later version (later releases should be backwards compatible).
My notes are slanted towards Windows OS but XviD will also work on Mac and Linux machines. Search the web for related downloads and info.
Note that if you have a much older (early development) version of XviD already on your machine you are advised to manually uninstall it first. Though note that development versions of XviD shouldn’t be on your machine unless you specifically installed them (they shouldn’t have crept on with any media player for instance). Try looking for XviD's own uninstall function in your program files in preference to uninstalling via OS default Add/Remove programs facilities (e.g. via Control Panel).