This might sound like a dumb question but here goes....
How can you tell if a file is copyrighted material. Are there any music files that are not copyrighted?

Hi--the question not dumb at all if you mean how to tell which files are OK to download and share. All files are automatically copyrighted or Public domain whenever they were created, but not all copyrighted files are allowed for sharing or downloading.

Currently, you can't be sure that a file is OK to share or download. It would be OK to share or download such files if the copyright holder has allowed this. See the examples on www.magnetmix.com for many examples.

How to know if files are authorized for sharing or downloading?

Some files have a license visible in the license column, but that is a new development. Most files don't have a license visible.

The past practise of thinking that all was public domain unless clearly indicated otherwise worked OK in the pre-digital age. Most people feel that provided they aren't selling such works it's OK.

Currently the copyright groups (such as the RIAA) are spending millions trying to get that idea changed around the world, and so there are attempts to try to flag files digitally.

So, until there is a clear way for unmarked files to be marked as OK (or not), you really do have to use your best judgement under the rules of your country.

Here is another dumb question: if I download a song without a license (if the download works, that is), can the FBI or something track down my IP address and sue me? That would really suck...