I'd love the power to control soundwaves. Not only could I issue whatever sound I want from myself, I could distort the sound waves around me to whatever I wanted as well. I could force people to end up saying things they didn't mean to, or make a cat talk like a human being. I could compose an entire song simply thinking about it, then play it from my mouth. I could imitate others' voices, which would make for some amazing prank calls. The list goes on.