i ran into a problem some 15 months ago in nov 2014 that i could no longer drop and drag keynote presentations into the keynote widget, soon after the latest release of keynote came out at that time if i remember correctly. i abandoned that upgrade when apple support told me to redo my entire book from scratch.
in july 2015, not wanting to get even further behind the software curve and as yet more new releases of software was coming out, i had to bite the bullet and upgrade everything. ran across this same problem and many more. july was lost in terms of productivity. the good news is just by chance i stumped upon a workaround by accident. i told apple the workaround and i would have hoped that it was fixed in subsequent releases. having finished my book and sent it off to the store, i'm going to upgrade yet again as soon as apple provides me with a fix to a strange problem that they are diligently working on and making progress with. it is a very interesting problem that i will discuss to the community soon. but back to you.
i got around the issue of drag and drop not working syndrome by having the file opened in keynote when i dragged the keynote to the keynote widget. i would try the following if i were you. open the movie in iMovie. then drag and drop the file to the multimedia widget. in fact, if i remember, you can bypass the widget by just drag and drop in the ibook author page. realize that most all my work is done inside keynote, so i can't say for sure if ib author works the same, but for sure i have drag and dropped audios onto an iba page, just not with the latest release of ib author. i use ib author in an unusual way. worth try.
if that does not work, then i would suggest you call the apple support line for ib author.