Pi (π) is the number given by dividing the circumference of a circle by the diameter. The melody of the PI symphony is based on this number, with 1 being the tonic, 2 being supertonic, 5 dominant, etc.. A zero is a wildcard note, and the rhythm and accompaniment are simply written to fit the melody.
The piece comprises four sections, each containing 50 digits of pi, that is, 50 notes in the melody. Each section is in a different key, with the note changing to reflect the key change. In the opening bars, the numbers are quite distinct.
π = 3.14159265358979323846264338327950288419716939937510 58209749445923078164062862089986280348253421170679 82148086513282306647093844609550582231725359408128 48111745028410270193852110555964462294895493038196...
As written by Brendan Petty: The Pi Song (MP3) and The Pi Song Sheet Music (PDF)
The Pi Song - Stage Band (MP3) and The Pi Song - Stage Band Sheet Music (PDF)
As interpreted by Josh Rosens... The Pi Song - Techno (MP3)