You’ve long been able to tell Siri to play any track on your iPhone, and the Say Play app gives you the same ability with music you don’t own. Just open the app, hit the red button and speak the name of the track.
To be sure, you could do a voice search in YouTube, but the app appears to simultaneously search several music video sources, and continues to play the track while you’re using other apps. As you’d expect from an app powered by Nuance, voice-recognition is faultless, and in my brief tests it was lightning-fast.
The app costs $0.99 on iTunes.