float number + music copyright

[Cruzore]

I have seen some scripts where a float number would be represented like 0.0f, but I have experienced that it also works without that "f". What does that f stand for, and when do you use it?

Now for the second question, when I want to use a song or a soundtrack made by someone else for my custom story, do I have to contact the maker and ask if I can use it, or would be using that song for my custom story only not break the copyright, as long as I give credits?