When theme music for * Close Encounters of the Third Kind* was played on radio in the late 1970s, there was a child’s voice saying “bye!” or something like that at the end. Is this available on YouTube or anywhere else online?
God help me, it’s got to be Meco’s “Theme from 'Close Encounters…,” which was even worse than his disco “Star Wars.” (The voice is at 4:06)
You owe me big time.
John Williams’s jazzy version was infinitely better.
I just listened to it…that was a delight. 
Memory is a funny thing, but I could have sworn that I heard Casey Kasem in is AT 40 broadcast say that the “child’s” voice was vocalized by Kvitka ‘Kasey’ Cisyk, who is obscurely known as the vocalist dubbed the song “You Light Up My Life” for Didi Conn in the movie of the same name. But I can’t find anything to corroborate my memory.
I just assumed (from reading the OP and never having heard the music) that it was s sound clip of the lit’lst abductee from the movie. Having listened to the end of the music, that view hasn’t changed.