We've always had microphones, but I've noticed that most of the time the groups that try to use them can't be heard very well (which lowers their score - if the judges can't hear you, they won't get your jokes or understand what your skit is about and will mark it down). The microphones are typically hung from above and really are only effective if you're standing within a certain section of the stage. So doing it without a recording is risky.
Pre-recorded is the preferred way to go to ensure that you can be heard, lines aren't forgotten, and the music / voices work together the way you want them to.
