Two experiments serve to examine how people make metacognitive judgments about the effects of task-irrelevant sounds on cognitive performance. According to the direct-access account, people have direct access to the processes causing auditory distraction. According to the processing-fluency account, people rely on the feeling of processing fluency to make heuristic metacognitive judgments about the distracting effects of sounds. To manipulate the processing fluency of simple piano melodies and segments of Mozart's sonata K. 448, the audio files of the music were either left in their original forward direction or reversed. The results favor the processing-fluency account over the direct-access account: Even though, objectively, forward and backward music had the same distracting effect on serial recall, stimulus-specific prospective metacognitive judgments showed that participants incorrectly predicted only backward music but not forward music to be distracting. The difference between forward and backward music was reduced but not eliminated in global retrospective metacognitive judgments that participants provided after having experienced the distracting effect of the music first-hand. The results thus provide evidence of a metacognitive illusion in people's judgments about the effects of music on cognitive performance.