THE WORDS IN A SONG ARE NOT THE "LETTERS"
Listening to English songs is a great way to learn English. I know quite a lot of students that have improved their English simply by listening to English songs over and over again. In this case, the secret is the repetition and the fact that by listening to a song hundreds of times you will (almost subconsciously) learn the words.
This is one of the reasons why learning English in England is so much easier than learning English in Spain. Anyway, the reason why Spanish people say "letters of the song" is clearly because in Spanish you say (letras). In English, when you are talking about this issue you need to say: "lyrics".
Hope that helps!
Soy el director de Daniel Smith + Partners. Tengo más de 15 años de experiencia enseñando inglés a españoles.