You could possibly get good results by targeting the address or data bus to/from the memory. Try grounding or crossing the memory bus traces to each other. This would scramble up the digital sample data being read, yet hopefully leave the sequencer and sample playback and os running untouched. You might get glitches, digital stuttering, short blocks of random samples being read from the wrong locations, random noises, could be pretty cool.
I have done similar on a sega megadrive, and not only did it replace certain game textures and sprites with random bits of other sprites, it did it only to separate layers o the character could still run around and jump and play the game, even if half the level was invisible or visually cut and pasted together all arseways. See video:
https://www.youtube.com/watch?v=XJUVh-XCQxMI don't think its m ntiened there but I was controlling this with a circuit bent keyboard, I just wired both devices to each others circuits somewhat randomly so pressing keys on the keyboard caused the megadrive to go nuts.