According to Jim S03 E17 Cheryl Sings

Cheryl tries to impress her boastful cousin Mindy by posing as the lead singer of Jim's band – even though she has a terrible voice. Jim agrees to perform at Mindy's engagement party with Cheryl playing the part in order to teach his wife a lesson about embellishment by making a fool of herself. However, when Mindy tells Jim that her fiancé, Eric, is a record producer, he must decide between telling Cheryl the truth and making a bad impression on Eric.