Lawsuit Filed To Prove Happy Birthday Is In The Public Domain
The copyrights were filed with the U.S. copyright office at some point.  If the copyright should be found invalid, does that obligate the former copyright holder to return licensing fees?  I somehow doubt it.  The licensor acted in good faith; if you now own a worthless license, that is not their fault.  Indeed, Warner themselves probably bought the copyrights from someone else (they were founded in 1958) believing them valid.  Willful fraud, on the other hand, is another matter.