I'll add a bit to @kerkeslager response in stating that a placebo is something that works because you are told it will work. Your example of death metal would only be a placebo affect if someone told you that listening to death metal will help your sleep and then it did. If you are just a hard core rocker and got used to listening to death metal to sleep and now you find that without it you don't sleep as well, that isn't the same thing as a placebo affect.

But I am interested in what a death metal sleeping app would actually look like. Of course I would have to ask if it's needed in the market place (does it create value), as opposed to just playing my Demented Ted playlist.