Answer:
You currently cannot re-enable them from within the game, but there is a workaround solution.
Exit the game if it's running.
Find your game folder. Make a backup copy of your file "medieval2.preference.cfg", then open the original file. If Windows asks you what program to use, choose Notepad.
In your medieval2.preference.cfg, you should find the category [video] and find the following lines:
assassination_movies = 1
infiltration_movies = 1
sabotage_movies = 1
event_movies = 1
You can enable a type of movie by setting it to 1 and disable it by setting it to 0.
