check the sfx folder has everything from the default build.
Or just put the whole PM build back on your SD card.
Comparing the sfx folder on my SD card with the base sfx folder, it was indeed missing files, so I just deleted it and replaced it with the base sfx folder. This fixed the problem with Mewtwo, so I thought all was well and good, but as I was playing Samus, her sounds progressively started to stop working--namely the sound of her double jump and her up B.
Go back into the sfx folder on my SD card and it's totally empty. Tried to copy all of the files from the base sfx folder to my sfx folder and it failed on some of them, so the folder is clearly corrupted, and continues getting corrupted somehow simply by playing the game.
This has been a recurring problem for me, and I don't know why. Has anyone else had this happen? If anyone can help with this, I'd appreciate it.