The bee currently residing in my bonnet

In my days in CD retail I'd frequently get irritated by customers coming in and telling me about this song they were after. Now do correct me if I'm wrong, but by my understanding, a song is a piece of music which is, by definition, sung - in other words, there has to be a human voice involved. Why then refer to any musical composition as a song?

Last night I saw pretty conclusive evidence of this being a wide-spread problem. I tend to sit through the end credits of most films and last night was no exception, having recognised a lot of the music in Casanova as Rameau and being curious to see whether there'd be confirmation of this. I'm still a little bemused over this preference for French baroque music considering the richness of Venetian music of the time, but was even more aghast to see the list of music used come under the heading "songs", despite vocal compositions only accounting for one or two of the forty or so pieces credited.

Now I realise that the vast majority of popular music (in the broadest meaning of the term) is vocal, but even those rare tracks which do not involve singing are referred to as instrumentals. (I'm not sure if this is meant as an adjectival noun or whether there is an implied, unspoken noun which follows.) For all that, no-one in their right mind could say that the word "song" and its various derivatives are beyond the comprehension of the average native English-speaker.

It's another case of lazy English, and if it's crept into film credits, I suspect this particular (ab)use of the language will prove very difficult, if not impossible, to correct. Oh well, more proof that I'm old-fashioned and pedantic ... as if you didn't know.