Pipes in command lines will redirect output to things and wouldn’t usually work as a separator between paths. Are you sure that’s what ffmpeg actually wants?
Anyway, I guess it works as expected, but I did not realize in the first run, because the output of ffmpeg did not succeed, but this is a different story.
When adding a pause command to the script, I can see that DO created a file list separated by the pipe char. So basically it worked, but not my ffmpeg command line
You’ll probably save yourself time by using scripting now rather than trying to do it with awful DOS for-loops and similar, only to give up and use scripting later.
If you can understand that DOS for-loop and its percent codes then basic JScript won’t be a problem and will be much more straightforward.