Here's one I've had for a little while: Say you're playing a sport with your friends. You're yelling at each other.. commands likes, "pass the ball here". Or if you're out playing paintball and trying to coordinate an attack - your sound, and your opponent's sound are both pretty important.

As far as I know, such use of voice does not exist in any game. Player's voice does not really interact with the environment. So, say someone says, "come to me!" through voice-chat.. you still have to look at the map to see where they actually are.