Valid Pikachu have index numbers between 0x00 to 0x29, hence glitch Pikachu cries may be accessed by playing 'entries' 0x2A and above.
Unlike some glitch audio glitch Pikachu cries often sound like static rather than being based on existing audio, but they may sometimes incorporate real Pikachu cries.
Methods of activating themUsing an arbitrary code execution setup redirected to item 3, the following item set up will enable the player to play a glitch Pikachu cry of their choice.
Items: Repel x (cry ID) Poké Ball x 6 Fresh Water x 33 Master Ball x 64 Soda Pop x 45 TM05 x 132 Lemonade x 201
ld e,xx ld b,3c ld hl,4000 call 3e84 ret
See also this thread on the Glitch City Laboratories forums.
Glitch Pikachu cries may play very rarely when encountering
Using the Xploder cheating device, pressing the activate switch on Pokémon Yellow's title screen shortly before pressing A and then repeatedly pressing A again (as to enter the Continue/New Game/Option or New Game/Option menu) after closing the menu can cause a glitch Pikachu cry to play.