One of the advantages Xbox Live has over the standard PC online gaming experience is that every player has a Communicator, a combined earpiece and microphone. This makes a real difference to the gameplay, but unless a player is very quick with the mute button, unintentional sounds are often leaked across the Internet.
For example, it's fairly common to hear young kids arguing with their parents about:
In certain circumstances, if the argument develops nicely, you may bear aural witness to a whack around the head and subsequent choked sobs before the player disconnects. Violence is also a likely outcome if a twelve year-old is shouting "Shit, a fucking sniper!" as a parent enters the room.
I've also played with someone whose parrot could be regularly heard addressing the team with various squawks. The Communicator is voice-activated, so anything nearby that's over a certain volume level will get through. Coughs, sneezes and sniffles are examples of very common (and genreally unwelcome) unintentional transmissions. Another regular intrusion is TV noise - a loud TV will transmit game noise such as gunshots or collisions through to everyone's headsets.
It may be interesting to speculate how long it will take for the first murder to be heard via Xbox Live, courtesy of a particularly aggrieved next of kin or a player too engrossed in Doom 3 to hear the back door being jimmied open...