It is an error to suppose that song is simply a set of words with music added. A written text is already a musical notation for a spoken one, and if an example of speech must be in some sense musical, and if it is not then it must be classified as silence. All utterance, therefore, is song, though we should point out that the written notation can be very vague, leaving almost everything to the reader.
Music composed as an accompaniment to a set of words transforms the existing music of the words, however varied that may be, while that which we conventionally call singing is an unusual rhythmic and tonal version of the words, and this can be provided by a voice alone, without the guidance of music proper. Thus, the correct definition of song is:
Utterance in which the musical pattern of speech is transformed by being diverted and marshalled by another, non-linguistic, or unusual linguistic pattern.
From this it is necessary to conclude that song is nothing very special.