Conversation so far:
How did they go about getting the voices of the chipmunks?
The Chipmunks' voices were recorded at half the normal tape speed.
Did they use different people for each of the chipmunks?
Ross Bagdasarian Sr.'s own voice.

Question: How did he come up with the idea?
Self-contained rewrite: How did Ross Bagdasarian Sr. come up with the idea for Alvin and the Chipmunks' voices?