If there are no copyrights to the song, then you can sell your own arrangements of it. But you can't rearrange, say, the Star Wars theme song and sell your arrangement without licensing, and good luck getting that. What song are you wanting to arrange, how old is it, and how are you so sure it's not public domain? It's funny how many people think that Happy Birthday is free and clear for the public to use in any way when it's actually got a tightly controlled copyright on it.