I was watching my tape of The Godfather Saga, the part where Deanna is drunk while dancing with Merle at Anthony's First Communion Party. A band of professional musicians is playing the dance music. Then I became aware of something I never noticed before despite my multiple viewings: While Rocco tells Fredo that he'll have to take care of Deanna, I hear a trill from a harmonica -- playing over the band, not as a part of it.

Just prior to that, when Fredo pulls Deanna aside and scolds her for being drunk, it looks like she has something long and silvery in her hand -- a harmonica?

I don't know if this harmonica sound is just in the saga and was edited out of the theatrical movie, but I'll paraphrase a quote from Jerry Seinfeld and ask, "Deanna's harmonica -- what's up with that?"