Madden NFL 13's voice-activated Kinect prompts work only in singleplayer offline modes and in online multiplayer. This probably is because the Kinect sensor can't differentiate voices in the same room—a shortcoming we've seen exploited to a hilarious end here in Skyrim. Yet, Skyrim also is less of a spectator sport than Madden, which is why the potential for voice-trolling here is large, if not as precipitous as attacking an entire village and then quicksaving.
"There's nothing you can do to defend against your brother," in the same room, Victor Lugo, a producer on Madden, told ESPN's Jon Robinson. "You'll just need to find a way to shut him up."
If you're playing in a high-traffic area, my suggestion would be to unplug the Kinect sensor, or deactivate its features, if that's available in the menu options when the game releases. Tiger Woods PGA Tour 13 didn't have an option to deactivate Kinect input. I draped a baseball cap over the thing or faced it backward to keep it from swapping menus when I leaned back in my desk chair.
For those in online multiplayer, don't worry. Once your team comes to the line "we mute you out for that 20 seconds before you snap the ball," Lugo said. "[I]t defends against you telling hike through and having it get picked up through their surround sound speakers."