My family did this as a test once a long time ago, because my dad is big into music and he wanted to show us. ^^
We turned the music up very high on the instrumental track, so loud that a person couldn't hear themselves when they sang.
Aother person wore the headphones that the vocal would play out of, so they could hear the vocal.
The result was always the same -- the person singing sang HORRIBLY because they couldn't hear themselves, and the person with the headphones was pretty much rolling on the floor laughing.
It was just hilarious!
So, normally, a performer will have an earpiece so that they can hear what they're sounding like.
