From what I understand, the game files contains the instruments only and it's the game code that tell it what to play it get the tune. This is because each time you level up Sona's abilities, it progressively adds more notes, or "layers" (of instruments? Sorry I'm not too good with music terms) to generate a (slightly) different track each time.
When DJ Sona was introduced, Riot announced that she has 3+ hours of recorded sounds. Someone showcases it here https://www.youtube.com/watch?v=c4SDFmFLvFM ... I suspect it's these 3 hours of sounds that form the soundtrack.
You have 2 choices now:
- Someone done an ingame record of all 3 tracks here: https://www.youtube.com/watch?v=mgLTNCMzd9Y
- Launch a custom game as DJ Sona and record the desired track yourself using 3rd party tools
I too prefer the in-game version as it's more smoother/softer for when I work.