To instantly buy all of the soundtracks of the game you must start a new game. After you have completed the mission on Mt. Gagazet, do not go to the missions on Besaid Island or the Zanarkand Ruins, go straight to Luca. Complete the mission there, then go back. Go to the Sphere Theatre and you should be able to buy the music and movie spheres with whatever money you have.

Movie Spheres - 2,000 Gil
Music Spheres - 1,000 Gil