It was heavily rumored that, by saying things like "Sony", "Playstation" or even "Sega" into the microphone, you could get Pikachu to become mad. However, it appears that this is in fact an urban legend, as there is no indication in the data file that these words were supposed to be registered. The few registered cases of an Angry Pikachu were probably just caused by the game not understanding what was said, which in turn led it to just pick a random word similar to what was heard.