With regards to songs... they can advance the narrative or, as in my case, they can supplement or illustrate the narrative. For example (made up because Iron Maiden is banned), I can put in "The Number Of The Beast" not because it's about a dream about the devil, but because it was the song playing when something happened. Just like the lyrics of "Lavender" by Marillion don't advance the narrative, but are the lyrics of the "love song" Fish sang about in Kayleigh.
I'ma try to get mine in tomorrow.