I've always incorrectly called any piece of music a "song" even if there are no vocals. Think it might be a Welsh thing in my case.
I'm first language Welsh and as far as I'm aware, the word for song is the same as the word for any piece of music. "Can" singular or "Caneuon" plural.
Seems a bit long winded to say "Darn o gerddoriaeth" ("A piece of music")