Hi! I just finished a project, but I’m using sounds from freesound.org which are under de creative commons by attribution 3.0 license Creative Commons — Attribution 3.0 Unported — CC BY 3.0
So I already know i can sell my game, because the license have no commercial restrictions, however i don’t know if additional procedures apply for copyrighting my game.
And other thing, to officially protect my game i must chose single application or standard application from the U.S ECO web site?
Copyright is a noun not a verb.
You do not “Copyright” something. Copyrights automatically accrue to the author of any significant artistic expression fixed in a tangible medium at the time of its creation.
What you might be confused by is registering copyright. In the United States, Registering gives you additional protections should you go to court and sue someone for violating your copyright. But you can sue someone without registering, its just that you cannot collect statutory damages in that case.
Now to answer your first question. You always have the Copyrights to anything you create. Where you are leveraging someone else’s work, what is created is called a “derivative work”. You hold the copyright to your part and the author of the other work or works hold the copyright to each of their parts.
The license agreement that came with those other works will tell you under what circumstances you can distribute their work. You need to meet all the requirements of all the third party licenses to legally distribute the combined derivative work.
When you register copyright in a derivative work, it is only the things you created that are registered.
To learn more about Copyright and derivative works, read this: