Conversation so far:
How did Bagdasarian go about getting the voices of the chipmunks?
He sped up the playback to create high-pitched voices.
Did Bagdasarian use different people for each of the chipmunks?
The voices of the group were all performed by Bagdasarian,

Question: How did he come up with the idea?
Self-contained rewrite: How did Bagdasarian come up with the voices for each chipmunk?