Mixing characters up with official characters of the Warcraft franchise is always a fun thing to do. Even better when it doesn't make lore nazis cringe. (Really, though, what doesn't?) But let's get on with the story itself.
"A commitment to something greater than themselves." That's one of the many slogans the Marine Corps uses to recruit. I see it weekly on a billboard on my way to class, right with a photo of a crisply-dressed marine. So what would be the equivalent of that phrase in the Warcraft universe? Dragon oaths, as noted in quests and lore, are not to be taken lightly. If you make an oath to a dragon, you'd best realize that there's no way you can back out of it. How, though, do dragons decide who must make an oath?
I had been thinking of Sintas' history one evening, and decided either she was on the front lines of the War of the Shifting Sands or she stayed home. I picked the former, figuring that was a better way to get her associated with Fandral.
However, there's a much more simplistic reason for why I decided to write a backstory for something like the flute. The flute's function is to allow Sintas to call on the aid of a dragon on any continent. Originally, it would call a blue, green, red, or bronze drake to her aid. However, since the blue dragonflight began the Nexus War and the green dragonflight ended up trapped in their own realm, my pickings were slimmed down. Plus... I don't have a blue or green drake in game as mounts. What I do have are red and bronze drakes, as well as nether drakes and a green proto drake. The flute gave me a reason to be able to interact with this many dragons as Sintas and use them in roleplay.
This story satisfied these two needs for Sintas' character: association with Fandral as well as dragons.