YouTube videos in Storyline

To actually put the video on a slide, you need to download it, which is making a copy, and only the copyright holder has the right (get it?, copyright - right to make a copy) to make a copy of it - not matter what your purpose is. (Well, there are a few purposes for which there are exceptions, but if you were one of the exceptions, you would know.)

So ethically, if you make a copy you need permission, but legally, who knows? Pick a bunch of attorneys, and start asking them, until you get an answer you like :). My experience suggests to me that  the law on this one is settled enough that they will all tell you that you are fine linking to youtube (inserting a web object), but that you cannot make a copy.