Hi, Morwen. We're not being rude. It's just that US copyright law is so complicated. Here's a summary:
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Works Originally Created and Published or Registered before January 1, 1978:

Under the law in effect before 1978, copyright was secured either on the date a work was published with a copyright notice or on the date of registration if the work was registered in unpublished form. In either case, the copyright endured for a first term of twenty-eight years from the date it was secured. During the last (twenty-eighth) year of the first term, the copyright was eligible for renewal. The Copyright Act of 1976 extended the renewal term from twenty-eight to forty-seven years for copyrights that were subsisting on January 1, 1978, or for pre-1978 copyrights restored under the Uruguay Round Agreements Act (URAA), making these works eligible for a total term of protection of seventy-five years. Public Law 105-298, also known as the "Sonny Bono Copyright Term Extension Act of 1998", was enacted on October 27, 1998, and further extends the renewal term of copyrights still subsisting on that date by an additional twenty years, providing for a renewal term of sixty-seven years and a total term of protection of ninety-five years.

Public Law 102-307, enacted on June 26, 1992, amended the 1976 Copyright Act to provide for automatic renewal of the term of copyrights secured between January 1, 1964, and December 31, 1977.

Public Law 102-307 makes renewal registration optional. Therefore filing for renewal registration is no longer required in order to extend the original twenty-eight year copyright term to the full ninety-five years. However, some benefits result from making a renewal registration during the twenty-eighth year of the original term.
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It is likely that the works of Irving Berlin have been given all the copyright ptotection possible. If we suppose his estate went for the "full 95 years," then copyright for the Cuba song won't expire until Dec 31, 2015. Bad luck!