Spelling of title

The correct spelling is "souffle" - the noun for "breath" or, in this case, "murmur". You will see coeur spelled both ways, but the correct spelling is "cœur", where the "o" and the "e" are joined in what is called a "ligature". "Souffle au cœur" is the French medical term for "heart murmur", though I think that Malle intended his audience to take the phrase figuratively as well as literally.

The opening credits clearly spell the words:

le Souffle au Coeur

I've never yet encountered the word 'cur' in French. Might be an archaic word or perhaps a specialized term but if so, it doesn't strike me as something to be used in a movie title.

I suspect though that while you may have seen it spelled cur, that has to do with the correct spelling with the oe forming one single letter, simply wasn't used for lack of a correct azerty keyboard or not knowing the ascii code ... I don't know the correct ascii code either. Some webpages mess up or don't show some of the less used ascii codes.