3x30 - The Fairly OddParents

To win the lead role in the school musical with Trixie Tang, Timmy wishes that he had Chip Skylark's voice. Unfortunately for Chip, this causes him to have Timmy's voice, and his career quickly plummets. Not only that, but Timmy cannot unwish the wish because Cosmo and Wanda can only grant wishes given out in his voice.