Kinda hard to explain but I listened and heard that the Pokemon's cry changes when it's HP is not full and it enters battle. Also, its cry is different when it faints.
When a Pokémon is released from its Poké Ball, it will call out its cry, which consists of an electronically made "noise". It will also cry out when using certain moves, such as Growl, Roar, and Hyper Voice. If the Pokémon faints or comes into battle when injured during Generation III and later, the cry will sound, but it will be altered slightly. The cry is also changed noticeably when a Pokémon is sent into a battle with a status ailment. Pokémon with evolutionary relations to each other (such as Charmander and Charmeleon) will often have notably similar cries, though others may sound very different (such as Kirlia and Gallade). - Bulbapedia
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