HI Stefan, I ran into this predicament just this week for my work as a music supervisor tasked with creating a new re-record of the tone poem and I have an answer for you.
Back then, works copyright terms began when scores were published with a copyright notice. So although the score was completed and premiered in 1928, Warner Brothers first published the sheet music in 1929 and as such the publishing rights will not enter the public domain in the United States until January 1, 2025.
This has been confirmed by our musicologist and the Duke University Center for Public Domain.

On Thursday, September 10, 2015 at 12:41:34 PM UTC-4, Stefan Huber wrote:
> I wonder if somebody can help me with the following question: George Gershwin's original shellac recordings are in the public domain in Europe, Canada, etc. because of the composer's death in 1937 and the fact that these recordings have been released more than fifty years before 2013. This of course does not include the recordings which feature vocals by Fred Astaire (because the lyrcis are still under copyright) and the orchestral version of "Rhapsody in Blue" (orchestrated by Ferde Grofé).
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> "An American in Paris" seems to be a different case: Gershwin composed and orchestrated it. Yet, most (all?) available recordings are based on a revised version by F. Campbell-Watson. Taking into account that Mr. Campbell-Waton most likely survived George Gershwin by a long time, some of the available recordings of "American" may be in the public domain due to the fifty-years-rule, but the score they are based on is not (which would require that all authors have been dead for more than 75 years). This brings me to my real question: Nathaniel Shilkret recorded the piece in 1929 for RCA Victor (with Gershwin on the celesta). Is this recording using the original orchestration/version and thus "fully" in the pbulic domain?
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> Sorry for the long message, but maybe somebody here can help me out. Any help would be really appreciated...