Years ago "mike" was short for microphone. In recent years "mic" has replaced it. What brought about the change? "mike" is obviously pronounced with a long "i", while "mic" would ordinarily be pronounced with a short "i" like in "tic". In general to get a long "i" an "e" is added to the end, but can't be done here, because the "c" then gets an "s" sound rather than "k" sound.