Better than a password, I would venture to say that this would closer resemble a public/private key system. Suppose the bank could run several tests, and learn

Better than a password, I would venture to say that this would closer resemble a public/private key system. Suppose the bank could run several tests, and learn the tendency with which your ear reacts, rather than recording a single response. I am making some assumptions about how the ear works, but I'm going to guess that it doesn't send the same response to every stimulus. For authenticating, the bank could then send a randomized sound every time, and listen for an expected response, which is in effect your ear's signature on the input sound. This way, a thief wouldn't be able to bypass the system by simply obtaining a recording of your ear; he would have to be able to mimic its responses.