To my surprise, the OED says both are valid uses, and "mike" actually might predate "mic". Its earliest example for mike is from 1926, while the earliest for mi

To my surprise, the OED says both are valid uses, and "mike" actually might predate "mic". Its earliest example for mike is from 1926, while the earliest for mic is 1961. You're on Air viii., 1926: "The man at the ‘mike’ watches..and tells the audience the type of play." Singers' Glossary of Show Business Jargon, 1961: "Microphone: (Abbr. mike or mic)."