First, you need to update. Current latest is 14 updates ahead of yours.
Second, UIPlaySound in NGUI 3 is what UIButtonSound was in NGUI 2. It's used to play sounds on click the same way you set it up. Update to the latest and drag in a Colored Button prefab into the scene from the Prefab Toolbar. It has UIPlaysound on it, which works.
If you can't hear a sound, check your main camera and the listener attached to it.