doesn't work for PowerPoint 2003, only for new pptx files (since 2007)

Seems like relative paths are supported, they are just not straightforward to implement. For example if your Powerpoint is in the Sdx folder, and the ss.avi is in the Media folder inside Sdx. The trick is to edit the xml files inside the Powerpoint file:

• Make a copy of your presentation as tyq.pptx (just to keep the original safe).

• Rename your Presentation.pptx file into tyq.pptx.zip

• Open the resulting zip file by clicking on it.

• It will open as a folder. Inside it, go to ppt, then slides, then _rels

• You will see files called slide1.xml.rels etc.

• Copy (drag and drop) all those to another place, outside the archive.

• Now open the file corresponding to the number of your slide with video with some editor (for example, Notepad).

• Look for the line that says something like: Target="file:///C:\Sdx\ss.avi"

• If your Powerpoint presentation is in the MyFiles folder, and the ss.avi is in the Media folder inside MyFiles, then change the above line to: Target="ss.avi"

• Save the slide1.xml.rels file (or whatever number it was), then drag it back to that archive (that is actually a Powerpoint file), and then rename the archive back to Presentation.pptx

• This is it. Now you should be able to carry around the folder MyFiles with the presentation and with the Media folder inside it that contains your video.

Works for me on Windows 10.

Good luck!