There are actually three types of copyrights that pertain to music like Clair de Lune.
The song itself is copyrighted, but having been published in 1905, Debussy's work is now in the public domain. However, that is not all you need to know.
Any time the work is printed, that printed sheet music is also copyrighted. It is okay to distribute your own reproduction of any original manuscript from the 1900s, but if you buy a manuscript in a music store that was printed recently, you cannot legally distribute it.
Any recording of a performance made recently is also protected by a "phonorecord" copyright. And to further complicate things, the recording presented in the movie and that found on the soundtrack album are two different works. I don't know if you're talking about the 1960 version or the recent film, but both are new enough that any music in the soundtrack is still protected.
You can, by the way, get free audio recordings and sheet music of this work. Clair de Lune is part of the mutopia project, and there are several very good performance recordings made under the creative commons license.