Either could be used. It refers to the words or lyrics of popular songs, which are preserved because people memorise them by singing the song (and teaching it to others). If it were not for this, the words of the song would be lost, because illiterate people can't write them down.

"letra
6. f. Conjunto de palabras puestas en música para que se canten, a diferencia de la misma música. La letra de una canción, de un himno, de una ópera."
http://dle.rae.es/?id=NAwHNtA