Leo thinks that John is incredibly lucky that his computer will start up and work at all after that, especially since John saw smoke coming out of the computer after it happened! Since it's only the audio he's having problems with, Leo thinks it may have just fried the sound card or even just the speakers. He could try disabling the computer's sound through BIOS to see if that stops the annoying screaming noise. Any connection into the wall is potentially a problem if lightning hits the building. Even appliances can get fried if lightning strikes.