The word "Engel" is singular and plural at the same time: der Engel: Singular die Engel: Plural In the song, it is implicitly implied that they speak about all angels (or angels in general). Hence they use the plural. (Note the line "Sie verzweifeln wie jeder andere". "Sie" is a clear plural marker.) By the way, it's a song of "Ben featuring Gim", see for instance http://www.youtube.com/watch?v=VoZlmEcACCc