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Does anyone know how I can make custom pokémon cries for pokered?
Also does anyone know how I can make custom pikachu cries for pokeyellow?
Editing the cries in general is very easy. Take a look at data/cries.asm
Adding the Pikachu cries from Pokemon Yellow would be much more complicated. It would require porting all the relevant code from pokeyellow to pokered.
It is possible, but probably not the best place to start for a beginner. You can always give it a shot. If you get stuck, post your specific questions here and someone here can try to assist.