Ken has written an article on reverse engineering the audio amplifier chip in the Nintendo Game Boy Color:
The Nintendo Game Boy Color is a handheld game console that was released in 1998. It uses an audio amplifier chip to drive the internal speaker or stereo headphones. In this blog post, I reverse-engineer this chip from die photos and explain how it works.1 It’s essentially three power op-amps with some interesting circuitry inside.
More details on Ken Shirriff’s blog.