Itunes missing files

Some songs I attempt to play display this message:

The song (song name) could not be used because the original file could not be found. Would you like to locate it?

I click locate and get the message "iTunes was not able to locate any of 572 missing files." "Files that could not be found are marked with a “!”."

any suggestions on how I might find these files? I have an iMac 10.6.8

thank you