Video or multi-media content must be given special consideration. The short of it is that you can play videos using syntax like the following from within your CHM file:

<<img start="FILEOPEN" loop="1" DYNSRC="images\Baja.wmv">>

where the video is compiled into the CHM file. You can also link to an external multimedia file on your Web site in the same way by using an external URL. The movie above will show immediately. To start the movie somewhat more interactively by mousing over you can use:

<<img start="MouseOver" loop="1" DYNSRC="images\Baja.wmv">>

Unfortunately MouseOver is the only event available in the Start attribute.

External Content - not an option for CHM files any longer

In many cases multi-media files are accessed through ActiveX controls or applets of some sort. The issue is that these controls cannot find files contained inside of the CHM file - so the multi-media file must be loaded externally from disk. This is more difficult than it sounds: You can't just ship the file right along with your CHM file because the Help file doesn't run out of the directory where the CHM lives and so finding external files is difficult. Even hardcoded paths in the file system don't work nowadays due to security restrictions in the browser.

Unfortunately there appears to be no longer a way to embed external files. The HHControl was designed for this for feeding the browser content out of a project relative directory, but due to the security lockdowns in recent versions of Windows this approach no longer works.

Your best bet for content like this is to link to an external page on a Web site and display the content from there.