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A later model had the firmware dumped via the piezo speaker http://www.ipodlinux.org/stories/piezo/

The archived version has the images that are referred to in the writing: https://web.archive.org/web/20050828114013/http://www.ipodli...

I wonder why they could only use the piezo buzzer and not send cleaner audio out of the headphone port.

The piezo was probably pretty close to a gpio line. There's normally a lot more setup involved getting true audio hardware going.