The Fairly OddParents S03E28 - Chip Off the Old Chip

To win the male lead in the school musical and play opposite Trixie Tang, Timmy wishes for Chip Skylark's voice. However, while Timmy has Chip's voice, Chip now has Timmy's voice, and Chip's singing career quickly plummets. Timmy tries to wish everything back to normal, but Cosmo and Wanda can only grant a wish made in their godchild's voice.