This is a difficult question. Now, I am not a lawyer, so do research. However, in my experience, the following is usually what goes into credits and permission for stuff like this:
If it goes back to before copyright laws, MAKE SURE that there is no copyright on it anyway. For instance, Nathaniel Hawthorne's works were created before modern copyright law. However, his estate (his family, I believe) did copyright his works. So, even if it was created before copyright laws existed, it still may have a copyright on it.
Secondly, as far as music goes - the copyright is going to belong not only to the artist, but to the people who perform it. For instance, while a symphony may not have to pay for rights to perform a requiem by Mozart, any person that wants to use THAT SPECIFIC RECORDING must get permission from the symphony. I know it sounds silly, but if the Boston Pops orchestra recorded the one you want to use, you would have to get their permission. However, I know that there are TONS of companies that offer free recordings of classical music. You just have to find it.
I hope this helps.
