I don't know how I missed it, but I clearly did and I don't have the book in front of me to find it... When did Rand find the song?? How on Earth did I miss this??? Can someone refresh my memory please?
He heard it being sung when he went through the pillars at Rhuidean. I'm unclear if he remembered it from then after he integrated LTT/all the past lives, or if it was the act of integration/acknowledging LTT's memories which gave him the knowledge.
There isn't just one song, though. There were several songs of Growing. That's one of the sad things about the Tinkers' search - they will never find "The Song" because the Song they are imagining doesn't exist.
RJ has confirmed that the song Rand heard in Rhuidean is the song that the Tuatha'an are looking for. However, yes, there are many growing songs, and the Tuatha'an have combined all of those songs into one mystical song that they'll never find, simply because it doesn't exist. But the individual songs do, and Rand knows one of them.