When shortening microphone, both mic and mike are standard, each with its pros and cons: mic is better for its similarity to the full word but worse for appearing to be pronounced as mick; mike is better for clearly indicating pronunciation but worse for its similarity to the common name and dissimilarity to the full word.
The phrase open mic is about eight times as common as open mike, but when used as a verb, miked up obliterates the goofy-looking miced up. Although mic is more pleasing to the eye as a clearly-trimmed version of the full word, mike makes more sense as a standardized and more-versatile shortening. Nevertheless, check your preferred style guide, which may indicate a preferred spelling.