[MX] To modify website, loaded movies with unknown origin

Hello,

I am modifying a website.
On a scene I have buttons (which are just graphics) that load movies (others scenes). The problem is that I can't find where this action has been written. And I would like to replace the previous movies with new ones. One new movie for each buttons.
Because the previous movies are useless now, they are not in the folder anymore. So there is the window "error opening" with the previous movies name.
Is that clear enough?

Thank you very much for your answer.