Towards the end of season four, Starbuck meets a piano player in the makeshift bar on Galactica.
Now, it appears that this guy is some kind of ghost/angel like Caprica-6 and the ghost Baltar - but I have an idea that he could also be the original copy of the seventh model (Daniel). It's mentioned right in the episode before that Cavil poisoned all of the bodies that were being prepared to accept a copy of Daniel - but it's ambiguous about the fate of the original cylon with that personality. Daniel was also mentioned as an 'artist' and 'sensitive' person.
So - could it have been Daniel that helped Starbuck realise what the music meant?