Pikachu's Cry in Pokémon Yellow Explained

How was the Game Boy able to play Pikachu's voice clips in Pokémon Yellow, and why does it not sound all that great? It's all explained right here.
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INLINE LINKS
Pokémon Yellow disassembly: https://github.com/pret/pokeyellow
HQ Pikachu Cry Pokémon Yellow patch (for English release): https://rgmechex.com/tech/HQPikachu.bps
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