Mixing content to tell a story can this be considered infringement?

Generally speaking what you want to do is infringement. It isn't possible to tell you whether it would be covered under any of the exceptions without actually seeing the work. Hire an intellectual property attorney.

I would take a look at the Center for Social Media's Fair Use Materials, including Best Practices for User-Generated Content. Courts have applied the doctrine of fair use to visual collages, but it all will turn on the extent of copying, the character of the new work, and whether the artistic expression of your work transforms the original materials. That said, I think audiovisual collages "Mashups" fall under the defense of fair use if properly done.

But remember, fair use isn't a right, it's a defense.

First, the amount of effort needed to create a work is irrelevant when considering whether the work is copyrightable or whether the use of pre-existing works to create the work is "fair" (and therefore not infringing). How hard it is to create the work (technically or physically or emotionally) does not matter. The touchstone is whether the finished work is sufficiently creative to justify copyright protection.

Second, a "mashup" video of the type that you describe is copyrightable and the creator owns the copyright in it. What the creator can lawfully do with that work, however, is another story.

Third, whether it is fair or not to use the unlicensed use of pre-exisiting works to create another depends entirely on the facts of each situation. It will, however, be a very rare case indeed (if ever) to fairly use two whole works to create a third.

Fourth, whether you can lawfully make a mashup video "in the first place" is akin to the question of whether a falling tree makes a sound in the forest if no one is there to hear it. Certainly you can make a mashup video. If no one but you sees it, however, is that an infringement? Existentially, probably yes. Is the "wrong" legally cognizable? No.

You can read what the US Supreme Court (the last word on the matter) has to say about parody here:
http://tr.im/uu9b

You can read more about copyright fair use here: http://fairuse.stanford.edu/