Continuity mistake: The microphone is taken by Eric Cartman away from the keyboard closer to the crowd. Another microphone appears back on top of the keyboard and then disappears. (00:13:45)

Continuity mistake: At several points during the final scene, the microphone stand disappears and reappears on the stage.
Answer: He appears in at least 5 other episodes, but usually just as a background character. He can be seen at Cartman's party in "Immaginationland". He was also one of the kids trying out to replace Butters in the "Professor Chaos" episode.
Bishop73