I remember those times when I get caught up in the music especially the orchestral soundtrack for a movie and then hear the same melody years later with matching lyrics that enhance it all the more.
Cavatina was a classical guitar piece composed by Stanley Myers and featured in The Deer Hunter, a film about the Vietnam war. It was a soothing melody in contrast to the stark violence in the said war film. I love listening to this played on guitar, piano, flute. And then Cleo Laine composed the lyrics to the melody and it became He was beautiful.
Gabriel's Oboe is from the film The Mission about Jesuit missionaries in South America. Jeremy Irons played the oboe in the movie while being approached by tribesmen. The soundtrack was composed by the incomparable Ennio Morricone.
Sarah Brightman of Phantom of the Opera fame repeatedly requested Morricone to allow her to turn the melody into a song, but the latter refused. After several requests from Brightman, the composer relented and with Chiara Ferraù providing the lyrics, the song is now called Nella Fantasia (In my Fantasy). Brightman was the first to record it and soon it was a staple piece in many opera crossover artists' repertoire.