Here's why:
* Alvin: Alvin has the highest pitched voice, often singing in a high falsetto.
* Simon: Simon's voice is higher than Theodore's, but lower than Alvin's. He has a more "normal" sounding voice for a chipmunk.
* Theodore: Theodore's voice is the deepest of the three, often described as a baritone or even a bass. He often sings the lower harmony parts in their songs.