Can't find where to edit

dashboard – appearance – widgets
???

Isn’t it in sidebar.php ?

it is a theme file
go to
dashboard – appearance – editor
click on the sidebar.php file in the right column
scan for the code

Do a ‘Search in files’ for a unique element it contains. Might give you a clue as to where it came from. In your case the id of the div: “multimedia_box”

Do you have a text editor like notepadd++? Use it to search through files within a directory.